The Influence of Thermal Properties Anisotropy on Subtractive Laser

materials

Article The Inﬂuence of Thermal Properties Anisotropy on Subtractive Laser Processing of B4C/h-BN Composites

Paweł Rutkowski * , Karol Gala, Kamila Misiura and Jan Huebner Department of Ceramics and Refractories, Faculty of Materials Science and Ceramics, AGH University of Science and Technology in Krakow, al. Mickiewicza 30, 30-059 Kraków, Poland; [email protected] (K.G.); [email protected] (K.M.); [email protected] (J.H.) * Correspondence: [email protected] or [email protected]

Received: 5 October 2020; Accepted: 10 November 2020; Published: 17 November 2020

Abstract: This work concerns boron carbide matrix composites with the addition of hexagonal boron nitride particles (h-BN) as a solid lubricate. The composite materials were hot-pressed and analysed in terms of phase, structure, and microstructure changes in relation to the h-BN content. The uniaxial pressure applied during the manufacturing process allowed the orientation of single h-BN particles and its agglomerates in perpendicular direction to the pressing axis. The anisotropy of heat transfer and thermal expansion coefficient (CTE) and density changes in relation to temperature are discussed. Thermal diffusivity and conductivity were measured in relation to the material direction by the laser flash analysis method (LFA). In this paper, understanding the heat flow and CTE changes allowed explaining the results of investigated subtractive laser processes of the manufactured composites. The laser ablation process was conducted on B4C/h-BN composites in parallel and perpendicular direction to each other. It was done in a continuous work (CW) mode at 50 W with a 40 µm spot and 3 mm/s beam travel speed. The influence of h-BN particles and their orientation on thermal properties is discussed. The effect of laser processing on B4C/h-BN composites was also discussed in relation to the material surface roughness measured with a confocal microscope, microstructure observations, density, and thermal properties changes in relation to the material direction.

Keywords: boron carbide; hexagonal boron nitride; anisotropy; thermal properties; solid lubricate; laser processing; ceramic matrix composite

1. Introduction The boron carbide is a low density material of 2.51 g/cm3 with rhombohedral crystallographic structure including a high melting point of about 2400 ◦C, very high hardness of 38 GPa, a good elastic, excellent wear and mechanical properties, and high neutron absorption cross section [1–4]. Due to these properties, it is widely used as sand blasting nozzles, armor plates, sliding rings, neutron-shielding materials in nuclear industry, milling agents, grinding material, hard coatings, and also boron neutron capture therapy (BNCT) [5–9]. The hexagonal boron nitride, also called white graphite, was very often investigated by researchers due to the possibilities to decrease the friction coefficient for cutting tools and bearing sliding applications [10,11]. This phase has been inserted into various oxides, nitrides, and carbides of advanced ceramic materials, for example: Si3N4, SiC, Al2O3,B4C, SiAlON. The first works concerning anisotropic properties of B4C-hBN ceramics were made by Robert Ruh in 1992 for 0, 20, 40, 60, 80, and 100 vol.% of hexagonal boron nitride. He noticed that there was a large anisotropy of thermal properties [12,13]. Even in the case of the hot-pressing process, a higher concentration of hBN will increase the material porosity and decrease the mechanical properties, which was confirmed by Li et al. They noticed that a 20% h-BN addition will improve the friction

Materials 2020, 13, 5191; doi:10.3390/ma13225191 www.mdpi.com/journal/materials Materials 2020, 13, 5191 2 of 23 coefficient of such composites and simultaneously, there is an increase in the wear coefficient [10,14–20] when 5 wt % of h-BN in the material is exceeded. They found that in distilled water conditions, the wear behavior of B4C/h-BN composites against steel is improved. In dry conditions, limiting the addition of h-BN keeps a good tribological performance of about 10%. In the research made by Ruh, there is a lack of composition between 0 and 20 and 40 vol.%, therefore, for the h-BN content, it is possible to keep a balance between the mechanical properties and thermal conductivity. The author of this paper investigated thermally and mechanically boron carbide 2–8 wt % of h-BN composites, which were hot-pressed with a chromium carbide sintering aid [21]. In the paper, the addition of h-BN did not decrease the thermal properties significantly, but instead, allowed keeping good values of mechanical properties at a similar or slightly higher level to the reference sample. The introduced 2D phase led to a decrease in the friction coefficient and wear rate. Based on the works by Ruh and previous research made by the authors of this paper, further research in the B4C/h-BN system was made and presented. For 0.5, 1, 2, 4, 8, 16, and 32 vol.% of the h-BN content the following anisotropic properties were examined in this paper: Linear thermal expansion coefficient, diffusivity, and thermal conductivity. The obtained thermal data were used to discuss an influence of the added 2D phase on the material cut shape quality after the laser ablation process under a CW mode laser processing. The correlation between thermal properties and laser ablation of boron carbide/h-BN composites provides new information crucial for understanding the rapid heat processing of this composite system and gives new data in the literature concerning the material behavior during laser processing.

2. Preparation and Examination Route To manufacture the boron carbide–hexagonal boron nitride composites, the following commercially available powders were used: Boron carbide grade HS nr AB134566 (97% purity, 2.51 g/cm3 density, and 0.8 µm average grain size) from the H.C. Starck company (Karlsruhe, Germany) and BO-501 (99.9% purity, 2.25 g/cm3 density, and 0.3–0.7 µm grain size) from the Atlantic Equipment Engineers company (Bergenfield, USA). No additional sintering agent was used. The powders were mixed together to obtain mixtures containing 0, 0.5, 1, 2, 4, 8, 16, 32 vol.% of h-BN phase, which fulfilled the gaps in Ruh mixtures’ compositions and further investigations. As the first step, the boron nitride powder was de-agglomerated in an ultrasonic bath in isopropyl alcohol for 4 h. During this step, boron carbide was added and the ultrasonic was used for 2 h in order to get a uniform powder mixture. Afterwards, the composition sets were homogenized for 20 h in a laboratory rotational dissolver, modelTD100 (Pendraulik-Teja) (Springe, Germany) with 900 rpm/min. Such prepared mixtures were dried using a magnetic stirrer with a heating up mode. This step took up to 3 h with 200 rpm/min. The phase compositions were checked by the XRD analysis (PANalitycal X-Ray Diffractor with X-Pert HighScore software) (Almelo, The Netherlands) and morphology by SEM observations (FEI Nova Nano SEM) (Brno, Czech Republic). The morphology examples of the powders are shown in Figures1 and2 and the powder compositions are collected in Table1. After the de-agglomeration of initial powders, as a result of the powder mixtures homogenization step, we observed two B4C-I and B4C-II boron carbide phases and also two h-BN-I and h-BN-II hexagonal boron nitride phases, which showed slightly different lattice parameters. Materials 2020, 13, 5191 3 of 23 Materials 2020, 13, x FOR PEER REVIEW 3 of 23 Materials 2020, 13, x FOR PEER REVIEW 3 of 23

Figure 1. Morphology of commercial boron carbide grade HS of H.C. Starck. FigureFigure 1. 1. MorphologyMorphology of of commercial commercial boron boron carbide grade HS of H.C. Starck Starck..

Figure 2. Morphology of the B4C—32 vol.% hexagonal boron nitride particles (h-BN) mixture. Figure 2. Morphology of the B4C—32 vol.% hexagonal boron nitride particles (h-BN) mixture. Figure 2. Morphology of the B4C—32 vol.% hexagonal boron nitride particles (h-BN) mixture.

Table 1. Phase composition of the B4C-h-BN mixture after the homogenization step. Table 1. Phase composition of the B 4C-h-BNC-h-BN mixture mixture after the homogenization step. Phase Composition of Mixtures (wt %) PhasePhase Composition Composition of of Mixtures Mixtures (wt(wt %) %) MaterialMaterial Material B4C-I B4C-II h-BN-I h-BN-II WC B4BC-I4C-I B 4C-II h-BN-I h-BN-I h-BN-II h-BN-II WC WC B4C 0 0 0 B4C 0 0 0 B4C 0 0 0 B4C/0.5 h-BN 47.0 50.7 1.1 1.1 0.1 B4CB/0.54C/0.5 h-BN h-BN 47.047.0 50.7 1.1 1.1 1.1 1.1 0.1 0.1 B4C/1 h-BN 46.6 49.5 1.6 2.1 0.2 B4CB/14C/1 h-BN h-BN 46.646.6 49.5 1.6 1.6 2.1 2.1 0.2 0.2 B CB/24C/2 h-BN h-BN 45.145.1 50.3 2.0 2.0 2.4 2.4 0.2 0.2 4 B4C/2 h-BN 45.1 50.3 2.0 2.4 0.2 B4CB/44C/4 h- BN h- BN 43.243.2 50.6 3.3 3.3 2.7 2.7 0.2 0.2 B4C/4 h- BN 43.2 50.6 3.3 2.7 0.2 B4CB/84C/8 h-BN h-BN 46.546.5 44.9 5.2 5.2 3.2 3.2 0.2 0.2 4 B4C/B16C/8 h-BN h-BN 40.946.5 44.9 44.1 5.2 10.2 3.2 4.6 0.2 0.2 B4C/16 h-BN 40.9 44.1 10.2 4.6 0.2 B4CB/324C/16 h-BN h-BN 34.140.9 44.1 42.0 10.2 4.4 4.6 19.4 0.2 0.1 B4C/32 h-BN 34.1 42.0 4.4 19.4 0.1 B4C/32 h-BN 34.1 42.0 4.4 19.4 0.1

TheThe prepared prepared granulated granulated mixtures mixtures were were hot-pressed hot-pressed using using a a Thermal Thermal Technology Technology LLC LLC HP HP The prepared granulated mixtures were hot-pressed using a Thermal Technology LLC HP apparatusapparatus (Santa (Santa Rosa, Rosa, CA, CA, USA) USA) at at 2150 2150 °C◦C under under 25 25 MPa MPa for for 30 30 min min in in argon argon flow. ﬂow. The The heating heating rate rate apparatus (Santa Rosa, CA, USA) at 2150 °C under 25 MPa for 30 min in argon flow. The heating rate waswas 10 °C/min◦C/min and the 25 mm diameter samples with 8 mm thickness were obtained. was 10 °C/min and the 25 mm diameter samples with 8 mm thickness were obtained. TheThe apparent apparent density density of of hot hot pressed pressed composites composites was was measured measured by the by hydrostatic the hydrostatic method. method. The The apparent density of hot pressed composites was measured by the hydrostatic method. The relativeThe relative density density was wascalculated calculated and the and average the average XRD XRDphase phase composition composition analysis analysis was taken was taken for this for relative density was calculated and the average XRD phase composition analysis was taken for this calculation. Measurements were made for three samples of each composition. Sinters were polished calculation. Measurements were made for three samples of each composition. Sinters were polished in different material directions using diamond and silica media. The phase composition was in different material directions using diamond and silica media. The phase composition was

Materials 2020, 13, 5191 4 of 23 Materials 2020, 13, x FOR PEER REVIEW 4 of 23 this calculation. Measurements were made for three samples of each composition. Sinters were investigated by XRD analysis on the sample cross-section and on its surface in perpendicular to the polished in different material directions using diamond and silica media. The phase composition was pressing direction. SEM observations were made on the samples’ surfaces in parallel and investigated by XRD analysis on the sample cross-section and on its surface in perpendicular to the perpendicular direction to the pressing axis, as shown on the example of laser treatment pressing direction. SEM observations were made on the samples’ surfaces in parallel and perpendicular investigations. The element distribution analysis was carried in order to identify phases in the direction to the pressing axis, as shown on the example of laser treatment investigations. The element material’s microstructure. distribution analysis was carried in order to identify phases in the material’s microstructure. Thermal diffusivity and thermal conductivity measurements were carried out using the laser Thermal diffusivity and thermal conductivity measurements were carried out using the laser flash flash analysis method on the Netzsch LFA apparatus (Selb, Germany). The material was investigated analysis method on the Netzsch LFA apparatus (Selb, Germany). The material was investigated at at 25 °C, 50 °C, 100 °C, 300 °C, 600 °C, and 900 °C in argon flow. The specific heat was calculated on 25 C, 50 C, 100 C, 300 C, 600 C, and 900 C in argon flow. The specific heat was calculated on the the◦ base of◦ the thermodynamic◦ ◦ data◦ available◦ in the literature [22]. base of the thermodynamic data available in the literature [22]. The confocal microscope Olympus Lext OLS4000 (Tokyo, Japan) was used to establish the The confocal microscope Olympus Lext OLS4000 (Tokyo, Japan) was used to establish the material material surface roughness (Ra), which can have an influence on laser beam energy absorption and surface roughness (R ), which can have an influence on laser beam energy absorption and laser laser treatment. The examineda materials were then taken under laser subtractive processing using treatment. The examined materials were then taken under laser subtractive processing using the the JKLaser 5000 (Rugby, UK) ytterbium doped fiber laser with a 1064 nm wavelength with a JKLaser 5000 (Rugby, UK) ytterbium doped fiber laser with a 1064 nm wavelength with a maximum maximum power of 200 W (JK200FL). The trials were made under the argon flow coming out from power of 200 W (JK200FL). The trials were made under the argon flow coming out from the standard the standard cutting head nozzle, where the argon pressure was set to 2 bars. The continuous wave cutting head nozzle, where the argon pressure was set to 2 bars. The continuous wave (CW) mode (CW) mode with a 50 W power laser beam was used to shape the composite material. The laser beam with a 50 W power laser beam was used to shape the composite material. The laser beam spot was spot was 40 microns. The process speed was set to 3 mm/s, with a single laser beam pass. The 40 microns. The process speed was set to 3 mm/s, with a single laser beam pass. The reference and reference and composite material was processed in parallel and perpendicular direction to the composite material was processed in parallel and perpendicular direction to the material pressing axis material pressing axis of hot-pressing. After the process, the laser treated material was investigated of hot-pressing. After the process, the laser treated material was investigated on the cross-section of on the cross-section of the laser cut surface by scanning electron microscopy (SEM). the laser cut surface by scanning electron microscopy (SEM). 3. Results and Discussion 3. Results and Discussion

3.1. Densification Densification and and Phase Phase Analysis Analysis The apparent apparent density density results results measured measured by by the the hydr hydrostaticostatic method method is presen is presentedted in Figure in Figure 3. The3. obtainedThe obtained data datashows shows that thatdense dense materials materials can canbe obtained be obtained only only with with the theaddition addition of up of upto 8 to vol.% 8 vol.% of hexagonalof hexagonal boron boron nitride, nitride, which which is ishigher higher than than the the literature literature data data [13]. [13]. A A further further increase increase of of h-BN content ledled toto aa decrease decrease in in the the material material density, density, which which results results from from porous porous agglomerates agglomerates of this of phase this phase(Figure (Figure4). The 4). density The density measurement measurement error increaseserror incr witheases the with h-BN the h-BN content, content, which which is connected is connected with withthe h-BN the h-BN distribution, distribution, water water adsorption adsorption to agglomerates, to agglomerates, and a possibleand a possible absence absence of h-BN of after h-BN the after HP thegraphite HP graphite foil removal. foil removal. The existence The existence of hexagonal of hexagonal boron nitride boron agglomerates nitride agglomerates can lead can to a lead decrease to a decreaseof the material of the frictionmaterial coe frictionfficient coefficient [21,23]. Such [21,23]. agglomerates Such agglomerates can strongly can adsorb strongly a large adsorb amount a large of amountwater from of water the atmosphere, from the whichatmosphere, was visible which on was the dilatometricvisible on the curve dilatometric changes as curve a negative changes jump as on a negativethe curve. jump Such on material the curve. required Such anmaterial additional required time inan theadditional vacuum time chamber in the in vacuum order to chamber remove the in orderadsorbed to remove water beforethe adsorbed the final water CTE before analysis, the whichfinal CTE is described analysis, inwhich a further is described part of thisin a paper.further part of this paper.

Figure 3. Densification of B4C/h-BN composites. Figure 3. Densiﬁcation of B4C/h-BN composites.

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Figure 4. Fracture SEM observation of B4C/16 vol.% h-BN composites. Figure 4. Fracture SEM observation of B4C/16 vol.% h-BN composites. Figure 4. Fracture SEM observation of B4C/16 vol.% h-BN composites. Due to the uniaxial pressure applied during the hot-pressing process, there should be a DueDue toto thethe uniaxial uniaxial pressure pressure applied applied during during the hot-pressingthe hot-pressing process, process, there should there beshould a diﬀ erencebe a difference in the XRD/Rietveld analysis measured in various material directions. XRD peak differencein the XRD /inRietveld the XRD/Rietveld analysis measured analysis in various measur materialed in directions.various material XRD peak directions. intensities XRD of selected peak intensities of selected material phases show various values in different material directions, which intensitiesmaterial phases of selected show variousmaterial values phases in dishowﬀerent various material values directions, in different which material have a translation directions, into which their have a translation into their content. The result of the phase composition analysis made in havecontent. a translation The result ofinto the their phase content. composition The analysisresult of made the inphase perpendicular composition (in-plane) analysis and made parallel in perpendicular (in-plane) and parallel (out-of-plane) direction to the pressing axis is presented in perpendicular(out-of-plane) direction(in-plane) to and the pressingparallel (out-of-plane) axis is presented direction in Figure to5 the. pressing axis is presented in Figure 5. Figure 5.

Figure 5. Phase composition analysis versus the B4C/h-BN composite direction. FigureFigure 5. 5. PhasePhase composition composition analysis analysis versus versus the the B B44C/h-BNC/h-BN composite composite direction. direction. The XRD/Rietveld analysis showed that for the hot-pressed material with up to 2 vol.% there is TheThe XRD/Rietveld XRD/Rietveld analysis analysis showed showed that that for for the the ho hot-pressedt-pressed material material with with up up to to2 vol.% 2 vol.% there there is almost no difference in the composite phase composition in relation to the material direction. The almostis almost no nodifference difference in the in thecomposite composite phase phase composition composition in relation in relation to the to material the material direction. direction. The change of the selected phase concentration was noticeable for composites with 8–16 vol.% of changeThe change of the of theselected selected phase phase concentration concentration was was noticeable noticeable for for composites composites with with 8–16 8–16 vol.% vol.% of of hexagonal boron nitride due to the 2D phase orientation in the material as a result of applied pressure. hexagonalhexagonal boron boron nitride nitride due due to to the the 2D 2D phase phase orientation orientation in in the the material material as as a aresult result of of applied applied pressure. pressure. For the highest content of h-BN solid lubricant, there was almost no difference due to the high content ForFor the the highest highest content content of of h-BN h-BN solid solid lubricant, lubricant, ther theree was was almost almost no no difference difference due due to to the the high high content content of even oriented h-BN agglomerates. In the case of a perpendicular direction, 1% of B2O3 was present ofof even even oriented oriented h-BN h-BN agglomerates. agglomerates. In In the the case case of of a a perpendicular perpendicular direction, 1% of B 2OO33 waswas present present in the samples with 32 vol.% of h-BN. It formed due to the oxidation of h-BN agglomerates. Some of inin the the samples samples with with 32 32 vol.% vol.% of of h-BN. h-BN. It It formed formed du duee to to the the oxidation oxidation of of h-BN h-BN agglomerates. agglomerates. Some Some of of the boron oxide can be present on the initial B4C powders grain surface, which cannot be detected thethe boron boron oxide cancan bebe presentpresent on on the the initial initial B 4BC4C powders powders grain grain surface, surface, which which cannot cannot be detectedbe detected due due to its low amount. It can happen, for example, during the cutting or polishing process of the dueto its to low its amount.low amount. It can It happen,can happen, for example, for example, during during the cutting the cutting or polishing or polishing process process of the sampleof the sample for XRD analysis. Also small quantity of tungsten borides were found with the used analysis samplefor XRD for analysis. XRD analysis. Also small Also quantity small quantity of tungsten of tungsten borides wereborides found were with found the with used the analysis used detectionanalysis detection level, about 0.4% in pure material and 1 vol.% h-BN composite. The WxBy tungsten boride detectionlevel, about level, 0.4% about in pure 0.4% material in pure and material 1 vol.% and h-BN 1 vol.% composite. h-BN composite. The WxBy Thetungsten WxBy boridetungsten is aboride result is a result of the reaction between boron carbide and tungsten carbide impurities. The identification isof a the result reaction of the between reaction boron between carbide boron and carbide tungsten and carbide tungsten impurities. carbide impurities. The identification The identification of the phase of the phase in the material microstructure was made by the SEM/EDS analysis and presented in ofin the materialphase in microstructurethe material mi wascrostructure made by the was SEM made/EDS by analysis the SE andM/EDS presented analysis in and Figure presented6. The EDS in Figure 6. The EDS analysis was made on the h-BN agglomerate in order to get a good element count. Figure 6. The EDS analysis was made on the h-BN agglomerate in order to get a good element count. The dark grey colour belongs to boron carbide and the light grey to hexagonal boron nitride. The dark grey colour belongs to boron carbide and the light grey to hexagonal boron nitride.

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analysis was made on the h-BN agglomerate in order to get a good element count. The dark grey Materialscolour belongs2020, 13, x toFOR boron PEER carbideREVIEW and the light grey to hexagonal boron nitride. 6 of 23 Materials 2020, 13, x FOR PEER REVIEW 6 of 23

Figure 6. SEM/EDS analysis and phase identification on an example of 16 vol.% h-BN composite Figure 6. SEM/EDS analysis and phase identification on an example of 16 vol.% h-BN composite microstructure.Figure 6. SEM /EDS analysis and phase identiﬁcation on an example of 16 vol.% h-BN compositemicrostructure. microstructure.

3.2.3.2.3.2. Microstructure Microstructure TheTheThe microstructure microstructuremicrostructure observations observationsobservations were werewere made mademade using usingusing scanning scanningscanning electron electronelectron microscopy microscopymicroscopy in inin parallel parallelparallel and andand perpendicularperpendicularperpendicular directiondirectiondirection to to theto thethe pressing pressingpressing axis ofaxisaxis the of HPof the process.the HPHP process. Theprocess. SEM polishedTheThe SEMSEM surface polishedpolished observations surfacesurface observationsmadeobservations in parallel mademade direction inin parallelparallel to the directiondirection pressing toto axis thethe areprepressing presentedssing axisaxis are inareFigures presentedpresented7 and inin8 . FiguresFigures The observation 77 andand 8.8. TheThe of observationreferenceobservation material ofof referencereference indicates materialmaterial the existence indicatesindicates of an thethe additional exexistenceistence “white” ofof anan phaseadditionaladditional in the “white”“white” microstructure, phasephase in whichin thethe microstructure, which was also noticed in the B4C/h-BN composites and can come from small WC wasmicrostructure, also noticed which in the was B4C /alsoh-BN noticed composites in the and B4C/h-BN can come composites from small and WC can impurities come from in small the initial WC impuritiespowders’impurities mixtures.in in the the initial initial The powders’ powders’ performed mixtures. mixtures. XRD phase The The performed performed composition XRD XRD and phase phase EDS composition composition element distribution and and EDS EDS element analysiselement distributionconfirmeddistribution a analysis formationanalysis confirmed confirmed of tungsten a a formation formation borides as of of a resulttung tungstensten of the borides borides tungsten as as a a carbide result result of reactionof the the tungsten tungsten with a materialcarbide carbide reactionmatrixreaction during with with a a thematerial material sintering matrix matrix process. during during Tungsten the the sinter sinter carbideinging process. process. was found Tungsten Tungsten in up carbide tocarbide 0.1% concentrationwaswas found found in in up inup theto to 0.1%material0.1% concentration concentration initial mixtures. in in the the material Thematerial tungsten initial initial boride mixtures mixtures was. .The aThe contamination tungsten tungsten boride boride in the was was quantity a a contamination contamination of up to 0.4% in in the the in quantitysinters.quantity Theof of up up images to to 0.4% 0.4% in Figuresin in sinters. sinters.7 and The The8 showimages images an in existencein Figures Figures of7 7 and upand to 8 8 20show showµm an an size existence existence agglomerates of of up up to to of 20 20 hexagonal µm µm size size agglomeratesboronagglomerates nitride for ofof hexagonal sampleshexagonal in boron theboron pressing ninitridetride axis forfor direction. samplessamples in Thein thethe content pressingpressing of agglomerates axisaxis direction.direction. increases TheThe contentcontent with the ofof agglomeratesrisingagglomerates concentration increases increases of thewith with introduced the the rising rising h-BN concentratio concentratio to the composites.nn of of the the introduced introduced The fracture h-BN h-BN images to to the the of composites. compositescomposites. withThe The fracture16fracture and32 imagesimages vol.% of of h-BNcompositescomposites presented withwith in 1616 Figures andand 32329 and vol.%vol.% 10 ofalsoof h-BNh-BN confirm presentedpresented some fine inin FiguresFigures h-BN particles 99 andand 1010/flakes, alsoalso confirm some fine h-BN particles/flakes, which are well distributed in the B4C matrix and oriented in whichconfirm are some well fine distributed h-BN particles/flakes, in the B4C matrix which and are oriented well distributed in one direction in the B in4C the matrix matrix and (Figure oriented 10). in one direction in the matrix (Figure 10). one direction in the matrix (Figure 10).

Figure 7. SEM surface observations of composites with 4 vol.% h-BN (in parallel direction). Figure 7. SEM surface observations of composites with 4 vol.% h-BN (in(in parallelparallel direction).direction).

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Figure 8. SEM surface observations of composites with 16 vol.% h-BN (in parallel direction). FigureFigure 8. 8. SEMSEM surface surface observations observations of of composites composites with with 16 16 vol.% vol.% h-BN h-BN (in (in parallel parallel direction). direction). Figure 8. SEM surface observations of composites with 16 vol.% h-BN (in parallel direction).

Figure 9. SEM fracture observations of composites with 16 vol.% h-BN. FigureFigure 9. 9. SEMSEM fracture fracture observations observations of of composites composites with with 16 16 vol.% vol.% h-BN. h-BN. Figure 9. SEM fracture observations of composites with 16 vol.% h-BN.

Figure 10. SEM fracture observations of composites with 32 vol.% h-BN. Figure 10. SEM fracture observations of composites with 32 vol.% h-BN. FigureFigure 10. 10. SEMSEM fracture fracture observations observations of of composites composites with with 32 32 vol.% vol.% h-BN. h-BN. In order to confirm the anisotropic character of manufactured composites, the ultrasonic In order to confirm the anisotropic character of manufactured composites, the ultrasonic measurementsIn order toof confirmlongitudinal the anisotropicwave velocity character in relation of manufactured to the material composites, direction were the done.ultrasonic The measurements of longitudinal wave velocity in relation to the material direction were done. The measurementsresults are shown of inlongitudinal Figure 11. Thewave ultrasonic velocity resultin relations show to the the anisotropy material directionof wave velocity were done. reaching The results are shown in Figure 11. The ultrasonic results show the anisotropy of wave velocity reaching results are shown in Figure 11. The ultrasonic results show the anisotropy of wave velocity reaching

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In order to conﬁrm the anisotropic character of manufactured composites, the ultrasonic measurements of longitudinal wave velocity in relation to the material direction were done. The results MaterialsMaterials 2020 2020, 13, 13, x, xFOR FOR PEER PEER REVIEW REVIEW 8 8of of 23 23 are shown in Figure 11. The ultrasonic results show the anisotropy of wave velocity reaching 32% for 32%32%32 vol.%for for 32 32 ofvol.% vol.% h-BN. of of Ith-BN. h-BN. can alsoIt It can can result also also fromresult result the from from porosity the the porosity porosity of h-BN of of agglomerates, h-BN h-BN agglomerates, agglomerates, where thewhere where shape the the ofshape shape pores ofofhas pores pores an importanthas has an an important important role. role. role.

4 FigureFigureFigure 11. 11. 11. Anisotropy AnisotropyAnisotropy of of oflongitudinal longitudinal longitudinal wave wave wave velocity velocity velocity of of of B B BC/h-BN44C/h-BNC/h-BN composites. composites. composites. 3.3. Thermal Properties and Thermal Stability 3.3.3.3. Thermal Thermal Properties Properties and and Thermal Thermal Stability Stability The determination of thermal conductivity by the direct thermal diffusivity analysis on a LFA TheThe determination determination of of thermal thermal conductivity conductivity by by th the edirect direct thermal thermal diffusivity diffusivity analysis analysis on on a a LFA LFA apparatusapparatusapparatus cannot cannot cannot be be be made made made without without without specific specific specific heat heat heat da da data,ta,ta, linear linear linear material material material dimension dimension dimension changes, changes, changes, and and and density density density changeschangeschanges by by by dilatometry. dilatometry. dilatometry. The The The calculated calculated calculated specific specific specific heat heat heat on on the onthe base the base base of of thermodynamic thermodynamic of thermodynamic data data data[22] [22] and [and22] the andthe calculatedcalculatedthe calculated density density density on on the the on base base the of baseof dilatometric dilatometric of dilatometric measurements measurements measurements are are shown shown are shown in in Figures Figures in Figures 12 12 and and 12 13. 13.and The The 13 . densitydensityThe density changes changes changes were were calculated calculated were calculated on on the the base onbase the of of th base the ethermal thermal of the expansion thermal expansion expansion coefficient coefficient coe measured measuredfficient measured in in various various in materialmaterialvarious directions, materialdirections, directions, which which is is whichpresented presented is presented in in Figure Figure in 14 Figure14 for for the 14the fortechnical technical the technical linear linear linearCT CTEE and CTEand in in and Figure Figure in Figure 15 15 for for 15 thethefor physical physical the physical linear linear linear CTE. CTE. CTE.The The conducted Theconducted conducted measurements measurements measurements and and calculations and calculations calculations showed showed showed a a slight slight a slight decrease decrease decrease of of materialmaterialof material density density density with with with the the theincreased increased increased temperature, temperature, temperature, which which which has has has a a negligible anegligible negligible influence influence influence on on on the the the thermal thermal thermal conductivityconductivityconductivity analysis. analysis.analysis. For ForFor future futurefuture purposes, purposes,purposes, the mathematical ththe e mathematicalmathematical functions functionsfunctions were fitted werewere to the fittedfitted experimental toto thethe experimentalexperimentaldata and collected data data and and in collected Tablecollected2. Figurein in Table Table 14 2. 2.shows Figure Figure a 14 similar14 shows shows behavior a a similar similar ofbehavior behavior density of of changes density density forchanges changes all of for thefor allallmanufactured of of the the manufactured manufactured polycrystalline po polycrystallinelycrystalline materials. materials. materials.

FigureFigureFigure 12. 12. 12. Calculated CalculatedCalculated specific specific speciﬁc heat heat heat versus versus versus temperat temperat temperatureureure on on on the the the base base base of of of NIST NIST NIST database database database [22]. [22]. [22 ].

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Figure 13. Experimental density changes versus temperature.

Table 2. Mathematical function of density changes fitted to the experimental data (T-temperature (°C)).

vol.% of h-BN Mathematical Function of Density Changes (g/cm3) Fitting 0 y = −9 × 10−9T2 − 3 × 10−5T + 2.5216 R² = 0.9998 0.5 y = −1 × 10−8T2 − 3 × 10−5T + 2.5192 R² = 0.9996 1 y = −9 × 10−9T2 − 3 × 10−5T + 2.5109 R² = 0.9997 2 y = −1 × 10−8T2 − 3 × 10−5T + 2.5064 R² = 0.9993 4 y = −1 × 10−8T2 − 3 × 10−5T + 2.4891 R² = 0.9998 8 y = −1 × 10−8T2 − 3 × 10−5T + 2.4491 R² = 0.9996 16 y = −1 × 10−8T2 − 3 × 10−5T + 2.3812 R² = 0.9996 32 FigureFigure 13. 13. ExperimentalExperimentaly = −8 × density10 density−9T2 − changes3 changes× 10−5T versus + versus2.3020 temperature. temperature. R² = 0.9999

Table 2. Mathematical function of density changes fitted to the experimental data (T-temperature (°C)).

vol.% of h-BN Mathematical Function of Density Changes (g/cm3) Fitting 0 y = −9 × 10−9T2 − 3 × 10−5T + 2.5216 R² = 0.9998 0.5 y = −1 × 10−8T2 − 3 × 10−5T + 2.5192 R² = 0.9996 1 y = −9 × 10−9T2 − 3 × 10−5T + 2.5109 R² = 0.9997 2 y = −1 × 10−8T2 − 3 × 10−5T + 2.5064 R² = 0.9993 4 y = −1 × 10−8T2 − 3 × 10−5T + 2.4891 R² = 0.9998 8 y = −1 × 10−8T2 − 3 × 10−5T + 2.4491 R² = 0.9996 16 y = −1 × 10−8T2 − 3 × 10−5T + 2.3812 R² = 0.9996 32 y = −8 × 10−9T2 − 3 × 10−5T + 2.3020 R² = 0.9999

FigureFigure 14. 14. MaterialMaterial technical technical thermal thermal expansion coecoefficiﬃcientent versus versus material material direction. direction. Empty Empty marks marks are arefor for the the perpendicular perpendicular direction direction to the to pressingthe pressing axis axis (in two-dimensional (in two-dimensional (2D) (2D) plane), plane), and ﬁlledand filled marks Materialsmarksare 2020 for are, the13 for, x pressing FOR the pressingPEER direction REVIEW direction (out-of-plane). (out-of-plane). 10 of 23

Figure 14. Material technical thermal expansion coefficient versus material direction. Empty marks are for the perpendicular direction to the pressing axis (in two-dimensional (2D) plane), and filled marks are for the pressing direction (out-of-plane).

FigureFigure 15. Material physical thermalthermal expansionexpansion coe coefficiﬃcientent versus versus material material direction. direction. Empty Empty marks marks are arefor thefor perpendicularthe perpendicular direction direction to the to pressing the pressing axis (in axis 2D (in plane), 2D plane), and ﬁlled and marks filled aremarks for theare pressing for the pressingdirection direction (out-of-plane). (out-of-plane).

The results of thermal expansion coefficient shown in Figures 14 and 15 indicate high anisotropy material linear changes versus material direction. A sudden increase of the technical and physical CTE value in the perpendicular direction begins for 0.5 vol.% of the h-BN content in the composite. A higher addition of h-BN does not significantly change the technical CTE. For the parallel direction to the pressing axis, it stays on the level of 5.4 × 10−6 1/K for 25–300 °C measurement range, 5.9 × 10−6 1/K for 25–600 °C measurement range, and 6.3 × 10−6 1/K for 25–900 °C measurement range. For the perpendicular direction to the pressing axis, it is as follows: 4.2 × 10−6 1/K for 25–300 °C measurement range, 4.9 × 10−6 1/K for 25–600 °C measurement range, and 5.3 × 10−6 1/K for 25–900 °C measurement range. The dilatometric analysis showed the material linear changes fluctuation in the case of physical CTE, which varied in relation to the h-BN content and especially the high temperatures visible for 32 vol.% h-BN at 900 °C. The calculated data of the technical and physical CTE anisotropy are illustrated in Figures 16 and 17. For high temperatures, the physical CTE anisotropy stays on the level of 50%. For lower temperatures, it is mostly between 20% and 30% (Figure 17). For the technical CTE, all the measured temperature ranges are between 20% and 30% (Figure 16).

Figure 16. Anisotropy of the technical thermal expansion coefficient.

Materials 2020, 13, x FOR PEER REVIEW 10 of 23

Materials 2020, 13, 5191 10 of 23

Table 2. Mathematical function of density changes ﬁtted to the experimental data (T-temperature (◦C)).

Mathematical Function of Density vol.% of h-BN Fitting Changes (g/cm3) 0 y = 9 10 9T2 3 10 5T + 2.5216 R2 = 0.9998 − × − − × − 0.5 y = 1 10 8T2 3 10 5T + 2.5192 R2 = 0.9996 − × − − × − Figure 15. Material physical1 thermaly expansion= 9 10 9 Tcoeffici2 3 10ent5T versus+ 2.5109 materialR 2di=rection.0.9997 Empty marks − × − − × − are for the perpendicular2 direction toy =the1 pressing10 8T2 3axis10 (in5T 2D+ 2.5064 plane), andR 2filled= 0.9993 marks are for the − × − − × − 4 y = 1 10 8T2 3 10 5T + 2.4891 R2 = 0.9998 pressing direction (out-of-plane). − × − − × − 8 y = 1 10 8T2 3 10 5T + 2.4491 R2 = 0.9996 − × − − × − 16 y = 1 10 8T2 3 10 5T + 2.3812 R2 = 0.9996 − × − − × − The results of thermal 32expansion coefficienty = 8 10 9 Tshown2 3 10 in5 FiguresT + 2.3020 14 andR 152 = indicate0.9999 high anisotropy − × − − × − material linear changes versus material direction. A sudden increase of the technical and physical CTE value in theThe perpendicular results of thermal direction expansion coebeginsfficient for shown 0.5 vol.% in Figures of the14 and h-BN 15 indicate content high in anisotropythe composite. A higher additionmaterial linearof h-BN changes does versus not significantly material direction. change A sudden the technical increase ofCTE. the technicalFor the andparallel physical direction CTE value in the perpendicular direction begins for 0.5 vol.% of the h-BN content in the composite. to the pressing axis, it stays on the level of 5.4 × 10−6 1/K for 25–300 °C measurement range, 5.9 × 10−6 A higher addition of h-BN does not significantly change the technical CTE. For the parallel direction to −6 1/K for 25–600 °C measurement range, and 6.3 × 106 1/K for 25–900 °C measurement range.6 For the the pressing axis, it stays on the level of 5.4 10− 1/K for 25–300 ◦C measurement range, 5.9 10− 1/K × 6 −6 × perpendicularfor 25–600 direction◦C measurement to the pressing range, axis, and 6.3it is as10 −follows:1/K for 4.2 25–900 × 10◦C 1/K measurement for 25–300 range. °C measurement For the −6 × 6−6 range, 4.9 perpendicular× 10 1/K for direction 25–600 to °C the measurement pressing axis, it isrange, as follows: and 4.25.3 ×10 10− 1 1/K/K for for 25–300 25–900◦C measurement °C measurement × range, 4.9 10 6 1/K for 25–600 C measurement range, and 5.3 10 6 1/K for 25–900 C measurement range. The dilatometric× − analysis showed◦ the material linear changes× − fluctuation in◦ the case of physical CTE, whichrange. varied The in dilatometric relation to analysis the h-BN showed content the material and linearespecially changes the fluctuation high temperatures in the case of physicalvisible for 32 CTE, which varied in relation to the h-BN content and especially the high temperatures visible for vol.% h-BN at 900 °C. The calculated data of the technical and physical CTE anisotropy are illustrated 32 vol.% h-BN at 900 ◦C. The calculated data of the technical and physical CTE anisotropy are illustrated in Figures in16 Figures and 17. 16 andFor 17high. For temperatures, high temperatures, the the physical physical CTE CTE anisotropy stays stays on theon levelthe level of 50%. of 50%. For lower Fortemperatures, lower temperatures, it is mostly it is mostly between between 20% 20% and and 30% 30% (Figure (Figure 17 17).). For For the the technical technical CTE, CTE, all the all the measured measuredtemperature temperature ranges ranges are between are between 20% 20% and and 30% 30% (Figure (Figure 16 16).).

Figure 16.Figure Anisotropy 16. Anisotropy of the of thetechnical technical thermal thermal expansion coe coefficient.ﬃcient.

Materials 2020, 13, 5191 11 of 23 Materials 2020, 13, x FOR PEER REVIEW 11 of 23

Figure Figure17. Anisotropy 17. Anisotropy of the of the physical physical th thermalermal expansionexpansion coe coefficient.fficient. In the literature, there is no excessive information concerning the influence of 0–32 vol.% h-BN and In thethe literature, applied pressure there of is the no hot-pressing excessive processinformat on theion anisotropy concerning of heat the transport influence versus of temperature0–32 vol.% h-BN and the appliedin the boron pressure carbide compositeof the ho matrix.t-pressing This is whyprocess the examples on the of anisot thermalropy diffusivity of heat data measuredtransport as versus temperaturea function in the of boron temperature carbide and composite material direction matrix. are presentedThis is why in Figures the examples 18 and 19. Forof lowerthermal contents diffusivity of h-BN, there is a slight increase of heat transfer due to the high material densification above 99%, low data measured as a function of temperature and material direction are presented in Figures 18 and content of flat defects (interphase boundary), and second 2D high conductive phase and low material 19. For loweranisotropy contents (Figure of 11h-BN,). A similar there situationis a slight occurs increase when using of heat a low tr quantityansfer due of graphene to the powderhigh material densification(GNP), above where 99%, a small low amount content of 2D of particles flat defects leads to an(interphase increase of thermalboundary), conductivity and second [23]. In the 2D high conductivecase phase of parallel and directionlow material to the pressinganisotropy axis (Figure(Figure 18 ),11). at lowerA similar temperatures situation diff usivityoccurs decreases when using a low quantitywith of the graphene h-BN addition powder higher than(GNP), 4 vol.%. where It is causeda small by amount a higher material of 2D particles porosity connected leads to mostly an increase with h-BN agglomerates, increasing the concentration of flat defects (fractures in Figures9 and 10) and of thermal conductivity [23]. In the case of parallel direction to the pressing axis (Figure 18), at lower orientation of 2D particles. The thermal conductivity of a platelet shaped h-BN is above 200 W/(m K) · temperaturesin the diffusivity plane direction decreases and around with 2 W the/(m h-BNK) through addition the plane higher direction than [24 4]. vol.%. The authors It is ofcaused [24] by a · higher materialconfirmed porosity that in theconnected plane orientation mostly ofwith h-BN, h- largerBN agglomerates, thermal properties increasing than in the the perpendicular concentration of flat defectsdirection (fractures make in the Figures 2D particle 9 plane.and 10) They and also orientation showed that veryof 2D small particles. particles andThe large thermal agglomerates conductivity of a platelettend shaped to orient h-BN less, therefore,is above it200 can W/(m also have∙K) anin influencethe plane on direction thermal properties, and around which 2 is W/(m its value∙K) inthrough the plane ourdirection case in perpendicular[24]. The authors direction of to[24] the confirmed pressing axis. that The in scattering the plane of phonon orientation at boundaries of h-BN, of larger the studied samples will cause the heat transfer to decrease mainly in perpendicular direction to the thermal properties than in the perpendicular direction make the 2D particle plane. They also showed pressing axis due to the material anisotropy. For higher temperatures (900 ◦C) all curves reach values that very betweensmall particles 2.3 and 2.6 and mm 2large/s. The agglomerates results of thermal tend diff usivityto orient measured less, therefore, in perpendicular it can direction also have an influence showon thermal higher values properties, for the higher which h-BN is contentits value in ain whole our rangecase ofin temperatures. perpendicular This direction parameter to the pressing axis.decreased The withscattering the temperature of phonon more slightlyat bounda thanries in the of parallel the studied direction. samples At 900 ◦C, will increasing cause the the heat 2 transfer toh-BN decrease content thermalmainly di inffusivity perpendicular changes from dire 2.4 toction 3.8 mm to /thes is visiblepressing in Figure axis 19 due. to the material anisotropy. For higher temperatures (900 °C) all curves reach values between 2.3 and 2.6 mm2/s. The results of thermal diffusivity measured in perpendicular direction show higher values for the higher h-BN content in a whole range of temperatures. This parameter decreased with the temperature more slightly than in the parallel direction. At 900 °C, increasing the h-BN content thermal diffusivity changes from 2.4 to 3.8 mm2/s is visible in Figure 19.

Figure 18. Thermal diffusivity versus temperature and h-BN content in the pressing direction.

Materials 2020, 13, x FOR PEER REVIEW 11 of 23

Figure 17. Anisotropy of the physical thermal expansion coefficient.

In the literature, there is no excessive information concerning the influence of 0–32 vol.% h-BN and the applied pressure of the hot-pressing process on the anisotropy of heat transport versus temperature in the boron carbide composite matrix. This is why the examples of thermal diffusivity data measured as a function of temperature and material direction are presented in Figures 18 and 19. For lower contents of h-BN, there is a slight increase of heat transfer due to the high material densification above 99%, low content of flat defects (interphase boundary), and second 2D high conductive phase and low material anisotropy (Figure 11). A similar situation occurs when using a low quantity of graphene powder (GNP), where a small amount of 2D particles leads to an increase of thermal conductivity [23]. In the case of parallel direction to the pressing axis (Figure 18), at lower temperatures diffusivity decreases with the h-BN addition higher than 4 vol.%. It is caused by a higher material porosity connected mostly with h-BN agglomerates, increasing the concentration of flat defects (fractures in Figures 9 and 10) and orientation of 2D particles. The thermal conductivity of a platelet shaped h-BN is above 200 W/(m∙K) in the plane direction and around 2 W/(m∙K) through the plane direction [24]. The authors of [24] confirmed that in the plane orientation of h-BN, larger thermal properties than in the perpendicular direction make the 2D particle plane. They also showed that very small particles and large agglomerates tend to orient less, therefore, it can also have an influence on thermal properties, which is its value in our case in perpendicular direction to the pressing axis. The scattering of phonon at boundaries of the studied samples will cause the heat transfer to decrease mainly in perpendicular direction to the pressing axis due to the material anisotropy. For higher temperatures (900 °C) all curves reach values between 2.3 and 2.6 mm2/s. The results of thermal diffusivity measured in perpendicular direction show higher values for the higher h-BN content in a whole range of temperatures. This parameter decreased with the temperature more slightly than in the parallel direction. At 900 °C, increasing the h-BN content thermal diffusivity changesMaterials from2020 2.4, 13 ,to 5191 3.8 mm2/s is visible in Figure 19. 12 of 23

Materials 2020,Figure 13, x FOR 18. ThermalPEER REVIEW diﬀusivity versus temperature and h-BN content in the pressing direction. 12 of 23 Figure 18. Thermal diffusivity versus temperature and h-BN content in the pressing direction.

FigureFigure 19. Thermal 19. Thermal diffusivity diﬀusivity versus versus temperature temperature and h- andBN h-BN content content perpendicularly perpendicularly to the to pressing the direction.pressing direction.

Concerning the pure nonstoichiometric boron carbide, the electronic transport causing polarons Concerning the pure nonstoichiometric boron carbide, the electronic transport causing polarons and phonon a play role in thermal conductivity and it was explained in NASA papers [25]. In our and phonon a play role in thermal conductivity and it was explained in NASA papers [25]. In our paper, we describe the B4C/h-BN composites thermal conductivity calculated on the base of thermal paper,diff weusivity, describe material the density,B4C/h-BN and composites specific heat thermal changes. conductivity Results of this calculated parameter on are the illustrated base of thermal in diffusivity,Figure 20 material for the parallel density, measurement and specific direction heat changes. to the pressing Results axis of (out-of-plane)this parameter and are in Figureillustrated 21 in Figurefor 20 the for perpendicular the parallel measurementmeasurement direction direction to to the the pressing pressing axis axis (in-plane). (out-of- Dueplane) to specificand in Figure heat 21 for thein mostperpendicular of the manufactured measurement composites, direction thermal to the conductivity pressing axis increases (in-plane). slightly Due in to the specific 25–50 ◦heatC in mostrange. of the The manufactured increase of thermal composites, conductivity thermal is caused conductivity by high conductiveincreases slightly 2D particles, in the high 25–50 material °C range. The densification,increase of andthermal lower conductivity 2D agglomerates is caused content. by For high higher conductive temperatures, 2D thereparticles, is a decrease high material of densification,this property and possibly lower 2D due agglomerates to phonon-phonon content. scattering For higher (Figures temperatures, 20 and 21). Thethere addition is a decrease of a low of this propertyamount possibly of h-BN due allows to obtainingphonon-phonon slightly about scattering 7% higher (Figures thermal 20 properties and 21). in The parallel addition direction of toa low the pressing axis than for the reference sample of 29.6 W/(m K). In the pressing direction, the higher amount of h-BN allows obtaining slightly about 7% higher ·thermal properties in parallel direction to decrease in thermal properties of this material with a temperature rise is also visible for composites the pressing axis than for the reference sample of 29.6 W/(m∙K). In the pressing direction, the higher with a higher h-BN content above 8 vol.%, which is marked by empty chart markers (Figure 20). Here, decreasewe have in thermal additional properties phonon scattering of this material at boundaries with a and temperature higher material rise is porosity, also visible which for influence composites withthe a higher lower heath-BN transfer content especially above 8 in vol.%, the pressing which direction.is marked The by thermalempty chart conductivity markers at (Figure 900 ◦C in 20). this Here, we havematerial additional direction phonon is in the levelscattering of 11.3–13.8 at boundaries W/(m K)—the and highest higher value material is for porosity, 2 vol.% h-BN which addition influence · the lowerand the heat lowest transfer for 32 especially vol.% of this in the phase. pressi Dueng to direction. the high materialThe thermal densities conductivity (Figure3), at in 900 the case°C in this materialof low direction h-BN concentrations is in the level the of decrease 11.3–13.8 of thermalW/(m∙K)—the properties highest in low value temperatures is for 2 vol.% can be h-BN explained addition and the lowest for 32 vol.% of this phase. Due to the high material densities (Figure 3), in the case of low h-BN concentrations the decrease of thermal properties in low temperatures can be explained by the finer microstructure following the h-BN addition, therefore, increasing flat defects (usually 2D particles are limiting the grain growth of matrix during the hot-pressing process). The fine material microstructure is presented in Figures 9 and 10. For higher temperatures, the phonon-phonon scattering and phase orientation mostly influence thermal conductivity. In thermal conductivity, the thermal barrier existing on the B4C/h-BN interface is very important, which was noticed by Ruh [13]. He stated that the mismatch in the elastic modulus of B4C and h-BN phases will cause phonon scattering. He also discussed that the microcracking or interracial adhesion can form a thermal barrier—which is in our case during the 2D particle orientation. Moreover, he added that Niihara discussed a possible thermal barrier on an example of Si3N4-BN composites. In our case, the higher amount of h-BN gives significantly lower thermal properties values, which is not only connected with an h-BN single particles/agglomerates orientation, but also higher porosity (lower density—Figure 3) existing in h-BN agglomerates. In the case of perpendicular direction, thermal conductivity values increase with the addition of a h-BN solid lubricant, which is visible for low temperatures up to 100 °C (Figure 21), but for higher temperatures they stay much higher than the reference sample. It can be explained by the h-BN particle orientation in perpendicular direction to the pressing axis (also confirmed by ultrasonic measurements) and agglomerates that the shape is elongated in this direction. The literature says that removing agglomerates or limiting their size can give a higher anisotropy, resulting in larger values of thermal properties in the 2D particle orientation [24], which

Materials 2020, 13, 5191 13 of 23 by the finer microstructure following the h-BN addition, therefore, increasing flat defects (usually 2D particles are limiting the grain growth of matrix during the hot-pressing process). The fine material microstructure is presented in Figures9 and 10. For higher temperatures, the phonon-phonon scattering and phase orientation mostly influence thermal conductivity. In thermal conductivity, the thermal barrier existing on the B4C/h-BN interface is very important, which was noticed by Ruh [13]. He stated that the mismatch in the elastic modulus of B4C and h-BN phases will cause phonon scattering. He also discussed that the microcracking or interracial adhesion can form a thermal barrier—which is in our case during the 2D particle orientation. Moreover, he added that Niihara discussed a possible thermal barrier on an example of Si3N4-BN composites. In our case, the higher amount of h-BN gives significantly lower thermal properties values, which is not only connected with an h-BN single particles/agglomerates orientation, but also higher porosity (lower density—Figure3) existing in h-BN agglomerates. In the case of perpendicular direction, thermal conductivity values increase with the addition of a h-BN solid lubricant, which is visible for low temperatures up to 100 ◦C (Figure 21), but for higher temperatures they stay much higher than the reference sample. It can be explained byMaterials the h-BN 2020, particle13, x FOR orientationPEER REVIEW in perpendicular direction to the pressing axis (also confirmed13 of by 23 Materialsultrasonic 2020 measurements), 13, x FOR PEER REVIEW and agglomerates that the shape is elongated in this direction. The literature13 of 23 sayscan thatbe a removingtask for future agglomerates research. orIn limitingthis direction, their size thermal can give conductivities a higher anisotropy, at 900 °C stay resulting above in 13 larger W/(m canvaluesK) bereaching a of task thermal for 18.3 future properties W/(m research.∙K) for in the theIn 2Dthis case particle direction, of the orientation highest thermal h-BN conductivities [24], co whichncentration. can at be 900 a The task°C staydecrease for above future of 13 research. thermal W/(m K)Inproperties thisreaching direction, 18.3in perpendicular thermalW/(m∙K) conductivities for thedirection case atof in 900the the highestC temper stay above h-BNature 13 cofunction Wncentration./(m K) is reaching less The aggressive decrease 18.3 W/(m ofthanK) thermal for in thethe ◦ · propertiescaseparallel of the case. highestin perpendicularFor a h-BN future concentration. investigation, direction Thein the the decrease fitted temper mathematical ofature thermal function properties functions is less inof perpendicularaggressivethermal conductivity than direction in the are parallelingiven the temperature in case. Table For 3. a future function investigation, is less aggressive the fitted than mathematical in the parallel functions case. of For thermal a future conductivity investigation, are giventhe fitted in Table mathematical 3. functions of thermal conductivity are given in Table3.

Figure 20. Thermal conductivity versus temperature and h-BN content in pressing direction. Figure 20. ThermalThermal conductivity conductivity versus versus temperature temperature an andd h-BN content in pr pressingessing direction.

FigureFigure 21. 21. ThermalThermal conductivityconductivity versusversus temperaturetemperature andand h-BNh-BN contentcontent perpendicularlyperpendicularly toto the the Figurepressingpressing 21. direction. direction. Thermal conductivity versus temperature and h-BN content perpendicularly to the pressing direction. Table 3. Mathematical function of heat transfer values changes fitted to the experimental data.

Table 3. Mathematical function Diffusivityof heat transfer mm values2/s changesConductivity fitted to the W/(m experimental K) data. vol.% 3 2 Direction DiffusivityA = −Aln(T) mm +2 /Bs Conductivityλ = AT + BT W/(m + CT K) + D vol.%h-BN Direction A =A − Aln(T) +B B A λ = AT3 +B BT2 + CTC + D D h-BN −9 −5 0 Parallel 2.801A 21.282B −1 A × 10 2 B× 10 −0.0351C 31.093D −9 −5 0 ParallelParallel 2.8013.029 21.28222.911 −1− 5× ×10 10−9 2 3× ×10 10−5 −0.0351−0.0417 31.09333.295 2 −9 −5 PerpendicularParallel 3.0293.149 22.911 23.780 −5− 1× ×10 10−9 3 2× ×10 10−5 −0.0417−0.0409 33.29534.366 2 −9 −5 PerpendicularParallel 3.1492.950 23.78022.372 −1− 2× ×10 10−9 2 2× ×10 10−5 −0.0409−0.0387 34.36632.156 4 −9 −5 PerpendicularParallel 2.9503.257 22.372 24.599 −2− 4× ×10 10−9 2 3× ×10 10−5 −0.0387−0.0449 32.15635.327 4 −10 −5 PerpendicularParallel 3.257 2.750 24.599 21.039 −4 2 × × 10 10−9 3 1× ×10 10−5 −0.0449−0.0305 35.32729.424 8 −9 −5 PerpendicularParallel 2.7503.140 21.039 24.124 2 −×5 10 × −1010 1 3× ×10 10−5 −0.0305−0.0397 29.42434.282 8 −9 −6 PerpendicularParallel 3.140 2.559 24.124 19.794 −5 5× × 10 10−9 3 2× ×10 10−5 −0.0397−0.0221 34.28226.678 16 −9 −6 PerpendicularParallel 2.559 3.395 19.794 26.243 5 6× ×10 10−9 2 9× ×10 10−6 −0.0221−0.0331 26.67835.631 16 −8 −5 PerpendicularParallel 3.395 2.066 26.243 16.418 6 1× ×10 10−9 9− ×1 10× 10−6 −0.0331−0.0086 35.63121.088 32 −8 −5 PerpendicularParallel 2.066 3.501 16.418 27.643 1 5× ×10 10−8 −1 6× × 10 10−5 −0.0086−0.0038 21.08834.929 32 Perpendicular 3.501 27.643 5 × 10−8 6 × 10−5 −0.0038 34.929 The material properties can change as a result of: Oxygen contamination in argon protective gas, highThe temperature, material properties long material can change treatment, as a resultand la of:ser Oxygenbeam processing contamination during in the argon LFA protective measurement. gas, high temperature, long material treatment, and laser beam processing during the LFA measurement.

Materials 2020, 13, 5191 14 of 23

Table 3. Mathematical function of heat transfer values changes ﬁtted to the experimental data.

Diﬀusivity mm2/s Conductivity W/(m K) vol.% Direction A = Aln(T) + B λ = AT3 + BT2 + CT + D h-BN − ABABCD 0 Parallel 2.801 21.282 1 10 9 2 10 5 0.0351 31.093 − × − × − − Parallel 3.029 22.911 5 10 9 3 10 5 0.0417 33.295 2 − × − × − − Perpendicular 3.149 23.780 1 10 9 2 10 5 0.0409 34.366 − × − × − − Parallel 2.950 22.372 2 10 9 2 10 5 0.0387 32.156 4 − × − × − − Perpendicular 3.257 24.599 4 10 9 3 10 5 0.0449 35.327 − × − × − − Parallel 2.750 21.039 2 10 10 1 10 5 0.0305 29.424 8 × − × − − Perpendicular 3.140 24.124 5 10 9 3 10 5 0.0397 34.282 − × − × − − Parallel 2.559 19.794 5 10 9 2 10 6 0.0221 26.678 16 × − × − − Perpendicular 3.395 26.243 6 10 9 9 10 6 0.0331 35.631 × − × − − Parallel 2.066 16.418 1 10 8 1 10 5 0.0086 21.088 32 × − − × − − Perpendicular 3.501 27.643 5 10 8 6 10 5 0.0038 34.929 × − × − − Materials 2020, 13, x FOR PEER REVIEW 14 of 23 The material properties can change as a result of: Oxygen contamination in argon protective gas, Thishigh temperature,is the reason long why material two treatment,extreme sample and laser compositions beam processing of the during reference the LFA boron measurement. carbide polycrystallineThis is the reason material why two and extreme one samplewith 32 compositions vol.% h-BN of were the reference taken unde boronr carbidethermal polycrystalline conductivity measurementsmaterial and one during with heating 32 vol.% and h-BN cooling were takensteps. underResults thermal of this conductivitycyclic experiment measurements are presented during in Figuresheating 22 and and cooling 23. The steps. obtained Results results of thisshow cyclic that experimentthere is almost are no presented influence in of Figures laser processing, 22 and 23 . temperature,The obtained resultsand gas show purity that on there the is thermal almost noprop inﬂuenceerties of laserboron processing, carbide based temperature, polycrystalline and gas materialspurity on with the thermal and without properties the h-BN of boron dispersed carbide phase. based The polycrystalline material is thermally materials stable with and in a without protective the atmosphere.h-BN dispersed phase. The material is thermally stable in a protective atmosphere.

Figure 22. Stability investigation of thermal diﬀusivity and conductivity of B4C reference material in Figure 22. Stability investigation of thermal diffusivity and conductivity of B4C reference material in parallel direction. The arrows indicate the direction of heating and cooling during the measurement. parallel direction. The arrows indicate the direction of heating and cooling during the measurement.

Figure 23. Stability investigation of thermal diffusivity and conductivity of B4C with 32 vol.% h-BN in parallel direction. The arrows indicate the direction of heating and cooling during the measurement.

For the onset of laser processing description, the heat transfer properties at room temperature are very important (Figure 24). During the process, material properties at higher temperatures become much more interesting. The change of thermal diffusivity and conductivity at 900 °C is illustrated in Figure 25, which is the approximate temperature that stays around the laser-treated place for a short time after the interaction between the laser and sample.

Materials 2020, 13, x FOR PEER REVIEW 14 of 23

This is the reason why two extreme sample compositions of the reference boron carbide polycrystalline material and one with 32 vol.% h-BN were taken under thermal conductivity measurements during heating and cooling steps. Results of this cyclic experiment are presented in Figures 22 and 23. The obtained results show that there is almost no influence of laser processing, temperature, and gas purity on the thermal properties of boron carbide based polycrystalline materials with and without the h-BN dispersed phase. The material is thermally stable in a protective atmosphere.

MaterialsFigure2020 22., 13 Stability, 5191 investigation of thermal diffusivity and conductivity of B4C reference material in15 of 23 parallel direction. The arrows indicate the direction of heating and cooling during the measurement.

FigureFigure 23. 23. StabilityStability investigation investigation of of thermal thermal didiffusivityﬀusivity andand conductivityconductivity ofof B B4C4C withwith 32 32 vol.% vol.% h-BN h-BN in inparallel parallel direction. direction. The arrowsThe arrows indicate indicate the direction the direction of heating of and heating cooling and during cooling the measurement. during the measurement. For the onset of laser processing description, the heat transfer properties at room temperature are veryFor important the onset (Figure of laser 24 processing). During the description, process, material the heat properties transfer properties at higher at temperatures room temperature become aremuch very more important interesting. (Figure The 24). change During of thermal the process, diﬀusivity material and conductivity properties at 900higher◦C istemperatures illustrated in becomeFigure 25much, which more is theinteresting. approximate The temperaturechange of thermal that stays diffusivity around the and laser-treated conductivity place at 900 for a° shortC is Materialstime after 2020, the 13, x interaction FOR PEER REVIEW between the laser and sample. 15 of 23 illustratedMaterials 2020 in, 13 Figure, x FOR PEER 25, whichREVIEW is the approximate temperature that stays around the laser-treated15 of 23 place for a short time after the interaction between the laser and sample.

FigureFigure 24. 24. ThermalThermal diffusivity diﬀusivity and and conductivity conductivity changes changes versus versus material material direction direction and and boron boron nitride nitride Figure 24. Thermal diffusivity and conductivity changes versus material direction and boron nitride contentcontent at at room room temperature temperature (RT). (RT). content at room temperature (RT).

FigureFigure 25. 25. ThermalThermal diffusivity diﬀusivity and and conductivity conductivity changes changes versus versus material material direction direction and and boron boron nitride nitride Figure 25. Thermal diffusivity and conductivity changes versus material direction and boron nitride contentcontent at at 900 900 °C.◦C. content at 900 °C.

The addition of hexagonal boron nitride will lead to an anisotropy of the thermal The addition of hexagonal boron nitride will lead to an anisotropy of the thermal conductivity/diffusivity as a result of the dispersed particles and pores orientation in the material conductivity/diffusivity as a result of the dispersed particles and pores orientation in the material microstructure. For room temperature (RT) conditions in the pressing axis direction, an increase of microstructure. For room temperature (RT) conditions in the pressing axis direction, an increase of h-BN particles will cause a strong decrease in thermal properties especially in the case of its higher h-BN particles will cause a strong decrease in thermal properties especially in the case of its higher h-BN content. A different situation occurs in the perpendicular direction where the heat transfer is h-BN content. A different situation occurs in the perpendicular direction where the heat transfer is faster. A similar situation can be met in high temperatures. There is a large thermal properties faster. A similar situation can be met in high temperatures. There is a large thermal properties anisotropy at low and high temperature which increased with the higher h-BN content in the anisotropy at low and high temperature which increased with the higher h-BN content in the composites, as clearly visible in Figures 24 and 25. This situation can have an influence on the material composites, as clearly visible in Figures 24 and 25. This situation can have an influence on the material treatment, which is presented in the next part of this paper. treatment, which is presented in the next part of this paper.

3.4. Subtractive Laser Processing 3.4. Subtractive Laser Processing There is some information in the literature about ceramic laser machining mostly in a pulse There is some information in the literature about ceramic laser machining mostly in a pulse mode and also in a continuous wave mode. The available example of an 8 mm thick ceramic tails mode and also in a continuous wave mode. The available example of an 8 mm thick ceramic tails laser cutting was shown by Black and Chua [26] in 1997. Their results confirmed that the pulse and laser cutting was shown by Black and Chua [26] in 1997. Their results confirmed that the pulse and continuous (CW) wave condition needs the multi-pass process, while in the case of CW, it was 60 continuous (CW) wave condition needs the multi-pass process, while in the case of CW, it was 60 passes, which is around 150 µm depth of a single cut. They were working with both the laser mode passes, which is around 150 µm depth of a single cut. They were working with both the laser mode and CO2 laser. For alumina it is 200 to 400 µm, and for a good quality silicon carbide it is up to 300 and CO2 laser. For alumina it is 200 to 400 µm, and for a good quality silicon carbide it is up to 300 µm in the laser pulse mode using the Yb:KGW laser (city, country) [27]. The laser ceramic treatment µm in the laser pulse mode using the Yb:KGW laser (city, country) [27]. The laser ceramic treatment depth was show also in publication [28]. However, there is no information concerning the laser depth was show also in publication [28]. However, there is no information concerning the laser cutting of anisotropic ceramic composites. In this study, the laser treated materials were taken into cutting of anisotropic ceramic composites. In this study, the laser treated materials were taken into the laser cutting process using a standard cutting laser head, argon flow, and continuous wave the laser cutting process using a standard cutting laser head, argon flow, and continuous wave

Materials 2020, 13, 5191 16 of 23

The addition of hexagonal boron nitride will lead to an anisotropy of the thermal conductivity/diffusivity as a result of the dispersed particles and pores orientation in the material microstructure. For room temperature (RT) conditions in the pressing axis direction, an increase of h-BN particles will cause a strong decrease in thermal properties especially in the case of its higher h-BN content. A different situation occurs in the perpendicular direction where the heat transfer is faster. A similar situation can be met in high temperatures. There is a large thermal properties anisotropy at low and high temperature which increased with the higher h-BN content in the composites, as clearly visible in Figures 24 and 25. This situation can have an influence on the material treatment, which is presented in the next part of this paper.

3.4. Subtractive Laser Processing There is some information in the literature about ceramic laser machining mostly in a pulse mode and also in a continuous wave mode. The available example of an 8 mm thick ceramic tails laser cutting was shown by Black and Chua [26] in 1997. Their results confirmed that the pulse and continuous (CW) wave condition needs the multi-pass process, while in the case of CW, it was 60 passes, which is around 150 µm depth of a single cut. They were working with both the laser mode and CO2 laser. For alumina it is 200 to 400 µm, and for a good quality silicon carbide it is up to 300 µm in the laser pulse mode using the Yb:KGW laser (city, country) [27]. The laser ceramic treatment depth was show also in publication [28]. However, there is no information concerning the laser cutting of anisotropic ceramic composites. In this study, the laser treated materials were taken into the laser cutting process using a standard cutting laser head, argon flow, and continuous wave working mode. The process was done and analyzed in two different material directions and for the material roughness (Ra) it was much lower and much higher than the wave length. For grinded materials with Ra around 2.69 µm, the observations are shown in Figures 26–31 and for polished composites with Ra around 0.56 µm, the observations are presented in Figures 32–37. A too low material roughness will lead to a light reflection. In this case, it gives a slightly better material treatment than in the case of a minimum of 2 times higher roughness towards the used laser wave length. In the case of a too high material (Ra), some energy absorbed by surface unevenness tends to generate some heat that is not involved in the material processing. Therefore, a too rough surface makes the effect of laser processing slightly lower in the perpendicular material direction than in the case of polished surface for a high amount of h-BN. For low concentrations of introduced hexagonal boron nitrides, there are visible cracks (Figures 28, 29, 31 and 35 ) or materials which are almost destroyed (Figures 27 and 34). Lots of cracks appear in the remelted areas (Figure 28). Cracks and material destruction during laser processing can appear as an effect of a large CTE difference between boron carbide and hexagonal boron nitride single phases (Figures 14–17). From the authors’ experience, similar cracks can possibly appear as an effect of h-BN decomposition and released nitrogen overpressure or in the case of residual water adsorbed in h-BN agglomerates. Materials 2020, 13, x FOR PEER REVIEW 16 of 23 Materials 2020, 13, x FOR PEER REVIEW 16 of 23 working mode. The process was done and analyzed in two different material directions and for the workingmaterial mode.roughness The process(Ra) it was was much done andlower analyzed and much in two higher different than materialthe wave directions length. For and grinded for the materialmaterials roughness with Ra around (Ra) it 2.69was µm,much the lower observations and much are higher shown than in Figures the wave 26–31 length. and forFor polishedgrinded materialscomposites with with R aR arounda around 2.69 0.56 µm, µm, the the observations observations are are shown presented in Figures in Figures 26–31 32–37.and for A polishedtoo low compositesmaterial roughness with Ra willaround lead 0.56 to aµm, light the reflection. observations In thisare case,presented it gives in Figuresa slightly 32–37. better A materialtoo low materialtreatment roughness than in the will case lead of a to minimum a light reflection. of 2 times Inhigher this roughnesscase, it gives towards a slightly the used better laser material wave treatmentlength. In thethan case in theof a case too highof a minimummaterial (R ofa), 2 some times energy higher absorbed roughness by towards surface unevennessthe used laser tends wave to length.generate In some the case heat of that a too is high not involvedmaterial (Rina ),the some material energy processing. absorbed byTherefore, surface unevennessa too rough tends surface to generatemakes the some effect heat of laser that processingis not involved slightly in lowerthe material in the perpendicularprocessing. Therefore, material directiona too rough than surface in the makescase of thepolished effect ofsurface laser forprocessing a high amount slightly of lower h-BN. in Forthe lowperpendicular concentrations material of introduced direction thanhexagonal in the caseboron of nitrides,polished theresurface are for visible a high cracks amount (Figures of h-BN. 28, For 29, low 31, concentrationsand 35) or materials of introduced which arehexagonal almost borondestroyed nitrides, (Figures there 27 are and visible 34). Lots cracks of cracks (Figures appear 28, 29, in 31,the andremelted 35) or areas materials (Figure which 28). Cracksare almost and destroyedmaterial destruction (Figures 27 during and 34). laser Lots processing of cracks can appear appear in as the an remeltedeffect of a areas large (FigureCTE difference 28). Cracks between and materialboron carbide destruction and duringhexagonal laser boro processingn nitride can singleappear phases as an effect (Figur of esa large 14–17). CTE Fromdifference the betweenauthors’ Materials 2020, 13, 5191 17 of 23 boronexperience, carbide similar and crackshexagonal can possiblyboron nitride appear single as an phaseseffect of(Figur h-BNes decomposition14–17). From andthe authors’released experience,nitrogen overpressure similar cracks or in can the casepossibly of re sidualappear water as an adsorbed effect of inh-BN h-BN decomposition agglomerates. and released nitrogen overpressure or in the case of residual water adsorbed in h-BN agglomerates.

Figure 26. SEM observations of a 50 W laser processed B4C reference sample (high surface roughness). Figure 26. SEM observations of a 50 W laser processed B4C reference sample (high surface roughness). Figure 26. SEM observations of a 50 W laser processed B4C reference sample (high surface roughness).

Figure 27. SEM observations of a 50 W laser processed B4C reference sample (high surface roughness). FigureFigure 27. SEM27. SEM observations observations of of a a 50 50 W W laserlaser processed B B4CC reference reference sample sample (high (high surface surface roughness). roughness). Materials 2020, 13, x FOR PEER REVIEW 4 17 of 23

Figure 28. SEM observations of a 50 W laser processed B4C—4 vol.% h-BN in pressing direction (high Figure 28. SEM observations of a 50 W laser processed B4C—4 vol.% h-BN in pressing direction (high surface roughness). surface roughness).

Figure 29. SEM observations of a 50 W laser processed B4C—4 vol.% h-BN in perpendicular direction (high surface roughness).

Figure 30. SEM observations of a 50 W laser processed B4C—8 vol.% h-BN in pressing direction (high surface roughness).

Materials 2020, 13, x FOR PEER REVIEW 17 of 23 Materials 2020, 13, x FOR PEER REVIEW 17 of 23

Materials 2020, 13, 5191 18 of 23 Figure 28. SEM observations of a 50 W laser processed B4C—4 vol.% h-BN in pressing direction (high surfaceFigure 28.roughness). SEM observations of a 50 W laser processed B4C—4 vol.% h-BN in pressing direction (high surface roughness).

Figure 29. SEM observations of a 50 W laser processed B4C—4 vol.% h-BN in perpendicular direction Figure 29. SEM observations of a 50 W laser processed B4C—4 vol.% h-BN in perpendicular direction (highFigure surface 29. SEM roughness). observations of a 50 W laser processed B4C—4 vol.% h-BN in perpendicular direction (high surface roughness). (high surface roughness).

Figure 30. SEM observations of a 50 W laser processed B4C—8 vol.% h-BN in pressing direction (high

FiguresurfaceFigure 30. SEM 30.roughness). SEM observations observations of of a a 50 50 W W laserlaser processed B B4C—84C—8 vol.% vol.% h-BN h-BN in pre inssing pressing direction direction (high (high surface roughness). Materialssurface 2020 roughness)., 13, x FOR PEER REVIEW 18 of 23

FigureFigure 31. SEM31. SEM observations observations of of a a 50 50 W W laserlaser processed B B4C—84C—8 vol.% vol.% h-BN h-BN in pe inrpendicular perpendicular direction direction (high(high surface surface roughness). roughness).

The increase in h-BN content above 8 vol.% causes the change of the laser cut shape, which is presented in Figures 30 and 31. For higher h-BN concentrations, the thermal anisotropy and h-BN agglomerates of the located porosity have a significant influence in laser processing. In perpendicular direction to the pressing axis (in-plane), the thermal diffusivity/conductivity is about 60% higher than in the pressing axis direction. It causes a heat transfer limitation in the out-of-plane direction and results in only 250–320 microns of processing depth for the rough surface sample. In this case, the ablation width is about 100 microns due to the heat transfer to the zone sides (Figure 32). There are no large cracks most likely due to the higher material porosity (Figure 3). Moreover, in the case of the low roughness polished sample with 32 vol.% h-BN, narrow, crack free, and deep laser cuts were observed (Figures 36 and 37). Due to the argon flow some material stays inside the cut, but it can be removed by use of short pulse laser processing and a vacuum or under pressure conditions. In most of the laser treatment cases, the anisotropy of thermal properties leads to deeper and narrower cuts. For a low concentration of h-BN, cuts have the shape of a droplet.

Figure 32. SEM observations of a 50 W laser processed B4C—32 vol.% h-BN in pressing direction (high surface roughness).

Materials 2020, 13, x FOR PEER REVIEW 18 of 23

Figure 31. SEM observations of a 50 W laser processed B4C—8 vol.% h-BN in perpendicular direction (high surface roughness).

The increase in h-BN content above 8 vol.% causes the change of the laser cut shape, which is presented in Figures 30 and 31. For higher h-BN concentrations, the thermal anisotropy and h-BN agglomerates of the located porosity have a significant influence in laser processing. In perpendicular direction to the pressing axis (in-plane), the thermal diffusivity/conductivity is about 60% higher than in the pressing axis direction. It causes a heat transfer limitation in the out-of-plane direction and results in only 250–320 microns of processing depth for the rough surface sample. In this case, the ablation width is about 100 microns due to the heat transfer to the zone sides (Figure 32). There are no large cracks most likely due to the higher material porosity (Figure 3). Moreover, in the case of the low roughness polished sample with 32 vol.% h-BN, narrow, crack free, and deep laser cuts were observed (Figures 36 and 37). Due to the argon flow some material stays inside the cut, but it can be removed by use of short pulse laser processing and a vacuum or under pressure conditions. In most Materials 2020, 13, 5191 19 of 23 of the laser treatment cases, the anisotropy of thermal properties leads to deeper and narrower cuts. For a low concentration of h-BN, cuts have the shape of a droplet.

Figure 32. SEM observations of a 50 W laser processed B4C—32 vol.% h-BN in pressing direction (high Figure 32. SEM observations of a 50 W laser processed B4C—32 vol.% h-BN in pressing direction (high surfacesurface roughness). roughness). Materials 2020, 13, x FOR PEER REVIEW 19 of 23 Materials 2020, 13, x FOR PEER REVIEW 19 of 23

Figure 33. SEM observations of a 50 W laser processed B4C—32 vol.% h-BN in perpendicular direction Figure 33. SEM observations of a 50 W laser processed B4C—32 vol.% h-BN in perpendicular direction Figure(high surface 33. SEM roughness). observations of a 50 W laser processed B4C—32 vol.% h-BN in perpendicular direction (high surface roughness). (high surface roughness).

Figure 34. SEM observations of a 50 W laser processed B4C—8 vol.% h-BN in pressing direction (low Figuresurface 34. roughness). SEM observations of a 50 W laser processed B4C—8 vol.% h-BN in pressing direction (low Figure 34. SEM observations of a 50 W laser processed B4C—8 vol.% h-BN in pressing direction (low surface roughness). surface roughness).

Figure 35. SEM observations of a 50 W laser processed B4C—8 vol.% h-BN in perpendicular direction Figure(low surface 35. SEM roughness). observations of a 50 W laser processed B4C—8 vol.% h-BN in perpendicular direction (low surface roughness).

Materials 2020, 13, x FOR PEER REVIEW 19 of 23

Figure 33. SEM observations of a 50 W laser processed B4C—32 vol.% h-BN in perpendicular direction (high surface roughness).

Materials 2020, 13, 5191 20 of 23 Figure 34. SEM observations of a 50 W laser processed B4C—8 vol.% h-BN in pressing direction (low surface roughness).

Figure 35. SEM observations of a 50 W laser processed B4C—8 vol.% h-BN in perpendicular direction Figure 35. SEM observations of a 50 W laser processed B4C—8 vol.% h-BN in perpendicular direction (low surface roughness). (lowMaterials surface 2020, 13 roughness)., x FOR PEER REVIEW 20 of 23 Materials 2020, 13, x FOR PEER REVIEW 20 of 23

Figure 36. SEM observations of a 50 W laser processed B4C—32 vol.% h-BN in pressing direction (low Figure 36. SEM observations of a 50 W laser processed B4C—32 vol.% h-BN in pressing direction (low surfaceFigure 36.roughness). SEM observations of a 50 W laser processed B4C—32 vol.% h-BN in pressing direction (low surface roughness). surface roughness).

Figure 37. SEM observations of a 50 W laser processed B4C—32 vol.% h-BN in perpendicular direction (lowFigure surface 37. SEM roughness). observations of a 50 W laser processed B4C—32 vol.% h-BN in perpendicular direction Figure 37. SEM observations of a 50 W laser processed B4C—32 vol.% h-BN in perpendicular direction (low surface roughness). (low surface roughness). The anisotropy of thermal properties, h-BN content, and material roughness also has an The anisotropy of thermal properties, h-BN content, and material roughness also has an influenceThe increase on the in width h-BN end content depth above of the8 material vol.% causes cut. Comparing the change samples of the containing laser cut 0, shape, 8, and which 32 is vol.%influence of h-BN on the in widththe case end of depth the material of the materialwith high cut. surface Comparing roughness samples which containing is higher 0, than 8, and laser 32 presented in Figures 30 and 31. For higher h-BN concentrations, the thermal anisotropy and h-BN wavelength,vol.% of h-BN the in cut the depth case changes of the material in following with way: high in surface parallel roughness direction towhich pressing is higher axis: 320 than µm laser for referencewavelength, sample, the cut 260 depth µm for changes 8 vol.% in h-BNfollowing and 310way: µm in parallelfor 32 vol.%. direction Therefore, to pressing in this axis: direction, 320 µm the for cutreference depth sample,is typical 260 as µmfor forthe 8ceramic vol.% h-BN materials. and 310 It changesµm for 32 in vol.%. perpendicular Therefore, direction: in this direction, 360 µm forthe reference,cut depth 400is typical µm for as 8 forvol.% the h-BN, ceramic and materials. 720 µm for It changes32 vol.% inh-BN. perpendicular The width direction:of the cut 360in the µm case for ofreference, pure boron 400 µmcarbide for 8can vol.% reach h-BN, 200 andmicrons 720 µm in thefor case32 vol.% of material h-BN. The destruction. width of Forthe cut8% inh-BN, the casethe widthof pure of boronthe cut carbide is around can 100 reach microns 200 microns in the pres in thesing case direction of material and 70 destruction. microns in theFor perpendicular 8% h-BN, the direction.width of the For cut 32%, is thearound width 100 of microns the cut is in around the pres 145sing microns direction in the and pressing 70 microns direction in the and perpendicular 40 microns indirection. the perpendicular For 32%, the direction. width of theBy cutanalyzing is around the 145 dimension microns in of the the pressing laser process direction ceramics and 40 microns with a surfacein the perpendicularroughness below direction. the beam By length, analyzing the reference the dimension sample ofshows the laserthe cut process around ceramics 300 µm inwith both a materialsurface roughness directions. below For the the high beam polished length, surface the reference in the casesample of 8 shows vol.% theh-BN cut addition, around 300 the µm measured in both depthmaterial is around directions. 230 Forµm theand high the widthpolished is 75 surface micron ins the in perpendicularcase of 8 vol.% direction h-BN addition, to the pressingthe measured axis, therefore,depth is around it is similar 230 µm to the and pressing the width direction. is 75 micron An interestings in perpendicular situation direction was noticed to the in thepressing case of axis, 32 vol.%therefore, of 2D it particles.is similar Into thisthe pressingcase, for direction.the well-polished An interesting surface situation the depth was is 150noticed µm higherin the case than of the 32 perpendicularvol.% of 2D particles. direction In (610 this µm).case, Thefor thedifference well-polished is also visiblesurface in the the depth width, is 150where µm 60 higher µm is than for thethe perpendicular direction (610 µm). The difference is also visible in the width, where 60 µm is for the

Materials 2020, 13, 5191 21 of 23 agglomerates of the located porosity have a significant influence in laser processing. In perpendicular direction to the pressing axis (in-plane), the thermal diffusivity/conductivity is about 60% higher than in the pressing axis direction. It causes a heat transfer limitation in the out-of-plane direction and results in only 250–320 microns of processing depth for the rough surface sample. In this case, the ablation width is about 100 microns due to the heat transfer to the zone sides (Figure 32). There are no large cracks most likely due to the higher material porosity (Figure3). Moreover, in the case of the low roughness polished sample with 32 vol.% h-BN, narrow, crack free, and deep laser cuts were observed (Figures 36 and 37). Due to the argon flow some material stays inside the cut, but it can be removed by use of short pulse laser processing and a vacuum or under pressure conditions. In most of the laser treatment cases, the anisotropy of thermal properties leads to deeper and narrower cuts. For a low concentration of h-BN, cuts have the shape of a droplet. The anisotropy of thermal properties, h-BN content, and material roughness also has an influence on the width end depth of the material cut. Comparing samples containing 0, 8, and 32 vol.% of h-BN in the case of the material with high surface roughness which is higher than laser wavelength, the cut depth changes in following way: in parallel direction to pressing axis: 320 µm for reference sample, 260 µm for 8 vol.% h-BN and 310 µm for 32 vol.%. Therefore, in this direction, the cut depth is typical as for the ceramic materials. It changes in perpendicular direction: 360 µm for reference, 400 µm for 8 vol.% h-BN, and 720 µm for 32 vol.% h-BN. The width of the cut in the case of pure boron carbide can reach 200 microns in the case of material destruction. For 8% h-BN, the width of the cut is around 100 microns in the pressing direction and 70 microns in the perpendicular direction. For 32%, the width of the cut is around 145 microns in the pressing direction and 40 microns in the perpendicular direction. By analyzing the dimension of the laser process ceramics with a surface roughness below the beam length, the reference sample shows the cut around 300 µm in both material directions. For the high polished surface in the case of 8 vol.% h-BN addition, the measured depth is around 230 µm and the width is 75 microns in perpendicular direction to the pressing axis, therefore, it is similar to the pressing direction. An interesting situation was noticed in the case of 32 vol.% of 2D particles. In this case, for the well-polished surface the depth is 150 µm higher than the perpendicular direction (610 µm). The difference is also visible in the width, where 60 µm is for the pressing direction and 40 µm for the perpendicular direction to the pressing axis. Here, it can be suggested that the reflectivity and energy absorptionMaterials 2020, rate13, x FOR should PEER REVIEW alsobe studied in the future. 21 of 23 Concerningpressing direction the chemical and 40 µm phenomena for the perpendicular taking direction place duringto the pressing laser axis. processing, Here, it can not be only the decompositionsuggested of that h-BN the reflectivity (about2500 and energy◦C) shouldabsorption be rate taken should under also be studied attention, in the butfuture. also the melting Concerning the chemical phenomena taking place during laser processing, not only the point of pure stoichiometric boron carbide (2450 ◦C) and eutectics in the system B4C-C (2375 ◦C) and decomposition of h-BN (about 2500 °C) should be taken under attention, but also the melting point B4C-B (2075of pure◦C) stoichiometric [29]. The confirmation boron carbide (2450 of liquid °C) and phase eutectics and in boronthe system carbide B4C-C formation(2375 °C) and in B4C- the cut tip is observedB in (2075 Figure °C) [29].32 and The revealedconfirmation by of the liquid EDS phase analysis and boron showing carbide onlyformation boron in andthe cut carbon tip is elements. The recrystallizedobserved in phase Figure is32 alsoand revealed visible inby Figurethe EDS 38analysis. showing only boron and carbon elements. The recrystallized phase is also visible in Figure 38.

Figure 38. Cross-section of B4C—1% h-BN process with 90 W. Figure 38. Cross-section of B4C—1% h-BN process with 90 W. 4. Conclusions During the research, anisotropic materials with a high relative density of over 94% were obtained by hot-pressing B4C with different amounts of h-BN addition. The laser processing conducted on all samples revealed a correlation between the h-BN content and shape of the performed cut. Additionally, the microstructure and thermal properties anisotropy is also dependent on the h-BN content in the samples. The porosity comes mostly from the h-BN agglomerates. A high anisotropy of the thermal expansion coefficient was observed between different material directions. It quickly reaches high values even for a low h-BN concentration, which can lead to material cracking during laser processing. The anisotropy of thermal conductivity and diffusivity was determined by LFA measurements. The thermal properties decrease in parallel direction to the pressing axis and slightly increase in perpendicular direction to the pressing axis. There is a high thermal properties anisotropy at low and high temperatures especially for the higher h-BN volume fraction. The observed high anisotropy of thermal properties influences the laser treatment of B4C/h-BN composites, resulting in deeper and narrower cuts after laser processing. The presence of a small amount of porosity in the material can protect composites from cracking during the laser action. The material surface roughness has little influence on laser processing and can be neglected. Future experiments employing the pulse working mode of laser and specific wave absorption coefficients are needed for a better understanding of the phenomenon during laser processing.

Author Contributions: All authors discussed and agreed upon the idea, and made scientific contributions. Conceptualization, P.R.; methodology, P.R., K.G., and K.M.; resources, P.R.; thermal expansion anisotropy investigation, P.R.; density changes versus temperature, P.R.; thermal diffusivity and conductivity, P.R. and K.G.; laser processing, P.R. and K.M.; other investigations, P.R., K.G., and K.M.; writing—original draft preparation, P.R.; writing—review, P.R.; writing—editing, P.R. and J.H.; visualization, P.R. and J.H.; supervision, P.R.; final approval, P.R. All authors have read and agreed to the published version of the manuscript.

Materials 2020, 13, 5191 22 of 23

4. Conclusions During the research, anisotropic materials with a high relative density of over 94% were obtained by hot-pressing B4C with different amounts of h-BN addition. The laser processing conducted on all samples revealed a correlation between the h-BN content and shape of the performed cut. Additionally, the microstructure and thermal properties anisotropy is also dependent on the h-BN content in the samples. The porosity comes mostly from the h-BN agglomerates. A high anisotropy of the thermal expansion coefficient was observed between different material directions. It quickly reaches high values even for a low h-BN concentration, which can lead to material cracking during laser processing. The anisotropy of thermal conductivity and diffusivity was determined by LFA measurements. The thermal properties decrease in parallel direction to the pressing axis and slightly increase in perpendicular direction to the pressing axis. There is a high thermal properties anisotropy at low and high temperatures especially for the higher h-BN volume fraction. The observed high anisotropy of thermal properties influences the laser treatment of B4C/h-BN composites, resulting in deeper and narrower cuts after laser processing. The presence of a small amount of porosity in the material can protect composites from cracking during the laser action. The material surface roughness has little influence on laser processing and can be neglected. Future experiments employing the pulse working mode of laser and specific wave absorption coefficients are needed for a better understanding of the phenomenon during laser processing.

Author Contributions: All authors discussed and agreed upon the idea, and made scientific contributions. Conceptualization, P.R.; methodology, P.R., K.G., and K.M.; resources, P.R.; thermal expansion anisotropy investigation, P.R.; density changes versus temperature, P.R.; thermal diffusivity and conductivity, P.R. and K.G.; laser processing, P.R. and K.M.; other investigations, P.R., K.G., and K.M.; writing—original draft preparation, P.R.; writing—review, P.R.; writing—editing, P.R. and J.H.; visualization, P.R. and J.H.; supervision, P.R.; final approval, P.R. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by the NCN statutory, project no. 16.16.160.557 of AGH University of Science and Technology in Krakow. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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