Microstructure and Properties of Tib2 Composites Produced by Spark Plasma Sintering with the Addition of Ti5si3

materials

Article Microstructure and Properties of TiB2 Composites Produced by Spark Plasma Sintering with the Addition of Ti5Si3

Agnieszka Twardowska 1,* , Marcin Podsiadło 2 , Iwona Sulima 1 , Krzysztof Bryła 1 and Paweł Hyjek 1

1 Institute of Technology, Pedagogical University, 2 Podchorazych, 30-084 Krakow, Poland; [email protected] (I.S.); [email protected] (K.B.); [email protected] (P.H.) 2 Łukasiewicz Research Network–Krakow Institute of Technology, 73 Zakopianska, 30-418 Krakow, Poland; [email protected] * Correspondence: [email protected]

Abstract: Titanium diboride (TiB2) is a hard, refractory material, attractive for a number of applications, including wear-resistant machine parts and tools, but it is difﬁcult to densify. The spark plasma

sintering (SPS) method allows producing TiB2-based composites of high density with different sintering aids, among them titanium silicides. In this paper, Ti5Si3 is used as a sintering aid for the sintering ◦ ◦ of TiB2/10 wt % Ti5Si3 and TiB2/20 wt % Ti5Si3 composites at 1600 C and 1700 C for 10 min. The phase composition of the initial powders and produced composites was analyzed by the X-ray diffraction method using CuKα radiation. The microstructure was examined using scanning electron microscopy, accompanied by energy-dispersive spectroscopy (EDS). The hardness was determined using a diamond indenter of Vickers geometry loaded at 9.81 N. Friction–wear properties were tested

in the dry sliding test in a ball-on-disc conﬁguration, using WC as a counterpart material. The major phases present in the TiB2/Ti5Si3 composites were TiB2 and Ti5Si3. Traces of TiC were also identiﬁed.

Citation: Twardowska, A.; The hardness of the TiB2/Ti5Si3 composites was in the range of 1860–2056 HV1 and decreased with Podsiadło, M.; Sulima, I.; Bryła, K.; Ti5Si3 content, as well as the speciﬁc wear rate Wv. The coefﬁcient of friction for the composites Hyjek, P. Microstructure and was in the range of 0.5–0.54, almost the same as for TiB2 sinters. The main mechanism of wear

Properties of TiB2 Composites was abrasive. Produced by Spark Plasma Sintering with the Addition of Ti5Si3. Materials Keywords: TiB2; Ti5Si3; spark plasma sintering; hardness; friction–wear properties 2021, 14, 3812. https://doi.org/ 10.3390/ma14143812

Academic Editor: Miguel Algueró 1. Introduction The most extensively studied phase in the Ti-B system is titanium diboride, as it is Received: 31 May 2021 the hardest and most thermodynamically stable. It is characterized by an extremely high Accepted: 5 July 2021 ◦ Published: 8 July 2021 melting point (3225 C), high hardness (35–40 GPa) and Young’s modulus (450 GPa), which are associated with relatively high thermal (60–120 W/mK) and electrical conductivities 5 Publisher’s Note: MDPI stays neutral (~10 S/cm) [1]. Due to this unique set of properties, titanium diboride is an attractive with regard to jurisdictional claims in material for a number of applications, including cutting tools, wear-resistant parts and published maps and institutional affil- coatings, ballistic armor, cathodes in Hall–Heroult cells for aluminum smelting, crucibles for iations. handling molten metals and metal evaporation boats [1,2]. TiB2 crystallizes in a hexagonal closely packed crystal lattice (P6/mmm space group) with elementary unit cell parameters of a = 3.02 Å and c = 3.22 Å. The anisotropy of its thermal expansion coefficient is believed to be a source of microcrack formation and brittleness, observed either at the synthesis stage of TiB or thermal cycles taking place afterward both in bulk materials and thin films [3]. Copyright: © 2021 by the authors. 2 Licensee MDPI, Basel, Switzerland. A low self-diffusion coefficient and an extremely high melting point make it difficult to This article is an open access article obtain TiB2 sinters of high density [4,5]. Among the different sintering methods, the most distributed under the terms and effective and fastest is spark plasma sintering (SPS). It was demonstrated that by the SPS conditions of the Creative Commons method, using simultaneously pulsed direct current and uniaxial pressure, TiB2 sinters Attribution (CC BY) license (https:// of 96% theoretical density could be produced within 10 min at a temperature range of ◦ creativecommons.org/licenses/by/ 1200–1800 C, but the produced sinters were susceptible to cracking. To limit the brittleness 4.0/). of TiB2 sinters, sintering aids have been introduced. The use of sintering additives is

Materials 2021, 14, 3812. https://doi.org/10.3390/ma14143812 https://www.mdpi.com/journal/materials Materials 2021, 14, 3812 2 of 11

advantageous for the density and temperature of sintering. Metal additives are effective in lowering the sintering temperature, but at the cost of lowering the mechanical properties. Among the ceramic aids used for TiB2 sintering are TiC [6], MoSi2 [7] and TiSi2 [8,9]. It was demonstrated that TiB2/TiSi2 composites with a TiSi2 content of 5–15 wt % produced by SPS at a temperature range of 1200–1400 ◦C can achieve high density (~98% of the theoretical density) [9]. It was found that the addition of TiSi2 enhances the densification, lowers the temperature of sintering and is beneficial for the mechanical properties of TiB2- ◦ based composites; however, due to the melting point of TiSi2, which is 1464 C, the working temperature of the produced composites is limited. There are four titanium silicides in the Ti-Si system, among them Ti5Si3 (Mn5Si3 crystal structure type), which is the most thermodynamically stable [10]. In comparison to TiSi2, Ti5Si3 is characterized by a lower density (4.32 g/cm3), a higher melting point (2130 ◦C), higher hardness and resistance to oxidation, so it is also an attractive material to be used as a sintering aid for TiB2-based composites [8]. Research on the use of this titanium silicide as a sintering additive is scarce because of its brittleness, which may adversely affect the properties of the obtained composites. Therefore, most studies concern the content of this silicide up to 5%. The aim of this study is to densify TiB2/Ti5Si3 micropowders by the SPS method and to study the influence of Ti5Si3 addition on the microstructure, hardness and friction–wear properties of the produced composites.

2. Materials and Methods 2.1. Powders, Powder Mixtures and Parameters of Their Homogenization

Two commercially available powders were used: TiB2 (Sigma-Aldrich, Darmstadt Germany) of 99.9 wt % chemical purity, grain size = <10 µm, density = 4.52 g/cm3; Ti5Si3 (Alfa Aesar, Kandel, Germany of 99.5 wt % chemical purity, grain size = <10 µm, 3 density = 4.32 g/cm . Two powder mixtures were investigated: TiB2 + 10 wt % Ti5Si3 and TiB2 + 20 wt. % Ti5Si3. The powder mixtures were homogenized three times in SpeedMixer TM DAC 400.1 FVZ (Hauschild & Co. KG, Hamm, Germany) at a rotational speed of 2000 RPM and a rotation time of 30 s, followed by a 10 s break.

2.2. Spark Plasma Sintering Spark plasma sintering was conducted using FCT Systeme GmbH (Effelder-Rauenstein, Germany), model HP D 5, and graphite die with an inner diameter of 20 mm. Densiﬁcation of powders started with initial degassing by pressing for 10 min in vacuum (5 × 10−1 mbar) to the maximum pressure of 35 MPa. After that, the furnace chamber was ﬁlled with argon, heated up at a rate of 200 ◦C/min to the selected sintering temperature (1600 ◦C and 1700 ◦C), held for 10 min then cooled down at a rate of 200 ◦C/min.

2.3. Relative Density, Microstructure Examination Sintered samples were mechanically ground and polished on one side using diamond grinding (9, 6 and 3 µm) and polishing suspensions (1 µm), respectively. At each grinding step, samples were degreased in isopropanol, ultrasonically cleaned in distilled water for 5 min and dried in air. The relative densities of the composites were determined by the Archimedes immersion method by measuring the weight differences of the specimens in air and in ethanol pure at room-temperature conditions. The theoretical densities of the samples were calculated according to the rule of mixtures. Phase compositions of powders and sinters were analyzed by the X-ray diffraction method (XRD) in Bragg–Brentano geometry using CuKα radiation (λ = 1.5406 Å, U = 40 kV, I = 30 mA). The XRD patterns were collected in 2θ geometry over scattering angles ranging from 20◦ to 90◦ with a step size of 0.02◦. Phase identiﬁcation was performed according to The International Centre for Diffraction Data (ICDD®) database. The microstructure of the samples was observed by the scanning electron microscope (SEM) JEOL JSM-6610 LV (JEOL Ltd., Tokyo, Japan) with energy-dispersive X-ray spectroscopy (EDS) using the X-Max detector (Oxford Instruments, Abingdon, UK), equipped with Aztec 2.1 software. Materials 2021, 14, x FOR PEER REVIEW 3 of 12

Materials 2021, 14, 3812 Japan) with energy-dispersive X-ray spectroscopy (EDS) using the X-Max detector3 of(Ox- 11 ford Instruments, Abingdon, UK), equipped with Aztec 2.1 software.

2.4. Hardness and Friction–Wear Tests 2.4. Hardness and Friction–Wear Tests Hardness was determined using the NEXUS 4000 hardness tester (Innovatest Europe Hardness was determined using the NEXUS 4000 hardness tester (Innovatest Europe bv, Maasstricht, The Netherlands) by the Vickers method using a diamond indenter bv, Maasstricht, The Netherlands) by the Vickers method using a diamond indenter loaded loaded at 9.81 N. Five indents were made in three different areas of the polished surface. at 9.81 N. Five indents were made in three different areas of the polished surface. The The friction–wear properties of the composites and referential TiB2 sinter sample of 97% friction–wear properties of the composites and referential TiB2 sinter sample of 97% relative densityrelative were density tested were in thetested dry in sliding the dry test sliding in a ball-on-disc test in a ball-on-disc geometry (Figure geometry1), according (Figure 1), toaccording the ASTM to G99-95athe ASTM standard G99-95a (reapproved standard (reapproved in 2000) [11 in]. As2000) a counterpart [11]. As a counterpart material, WC ma- (6terial, wt % WC Co) (6 was wt used. % Co) Balls was 3.175 used. mm Balls in diameter 3.175 mm were in loadeddiameter at 5were N. The loaded sliding at 5 velocity N. The wassliding 0.1 m/s,velocity while was the 0.1 total m/s, sliding while distancethe total wassliding 200 distance m. The test was duration 200 m. The was test 2000 duration s, and thewas diameter 2000 s, and of the the wear diameter track of was the 10 wear mm. track Friction–wear was 10 mm. tests Friction–wear were conducted tests were in air con- at ducted in air at a relative humidity of 40–45% at room-temperature conditions. The fric- a relative humidity of 40–45% at room-temperature conditions. The friction force Ft was measuredtion force continuouslyFt was measured during continuously the test using during an extensometer.the test using an For extensometer. each test, a new For WCeach balltest, was a new used. WC Before ball was each used. test, Before both the each ball te andst, both the the sample ball wereand the washed sample in were high-purity washed acetone,in high-purity dried in acetone, air and dried weighted. in air After and weig mountinghted. After in the mounting holder, the in ball the and holder, the samplethe ball wereand the washed sample again were (in washed ethanol) again then (in dried. ethanol) The coefﬁcient then dried. of The friction coefficient (CoF) wereof friction calculated (CoF) accordingwere calculated to Equation according (1): to Equation (1): CoF = Ft/Fn (1) CoF = Ft/Fn (1) where Ft is the measured friction force (N), and Fn is the load applied (N). where Ft is the measured friction force (N), and Fn is the load applied (N).

FigureFigure 1. 1.Ball-on-disc Ball-on-disc geometry geometry of of the the friction–wear friction–wear test. test.

TheThe speciﬁc specific wear wear rate rate of of the the specimens specimens (disc (disc and and ball) ball) was was calculated calculated from from the the mass mass differencedifference of of each each specimen specimen before before and and after after the the ball-on-disc ball-on-disc tests, tests, according according to Equation to Equation (2): (2): ma − mb Wv = – (2) W Fn × d × L (2) 3 wherewhere W Wv vis is the the speciﬁc specific wear wear rate rate (mm (mm/Nm),3/Nm), m maais is thethe mass mass before before the the test test (kg), (kg), m mb bis is the the 3 massmass after after the the test test (kg), (kg), F Fnnis is the the applied applied load load (N) (N) and and d d is is the the theoretical theoretical density density (kg/m (kg/m3).). TheThe microstructure microstructure of of the the composites composites was was observed observed using using SEM SEM before before and and after after the the friction–wearfriction–wear test. test. Microstructure Microstructure observations observations were were accompanied accompanied by by EDS EDS analysis. analysis.

3.3. ResultsResults 3.1. SPS Sintering 3.1. SPS Sintering The variation of temperature, applied force (pressure), punch displacement (piston The variation of temperature, applied force (pressure), punch displacement (piston movement) and sintering speed during the SPS cycle is shown in Figure2. The observed movement) and sintering speed during the SPS cycle is shown in Figure 2. The observed changes in the piston displacement determine the ﬁve stages of changes that take place changes in the piston displacement determine the five stages of changes that take place during sintering (I–VI) to represent the accompanying phenomena. In Stage I, particle rearrangementduring sintering takes (I–VI) place to represent initiated bythe the accomp appliedanying force. phenomena. A positive punchIn Stage displacement I, particle re- isarrangement recorded (Stages takes place I and initiated III). In between by the applied (Stage force. II), degassing A positive of punch the powder displacement mixture is under constant temperature and force takes place. In Stage IV, a slight negative piston displacement is recorded, caused by a temperature increase from 400 ◦C up to the requisite ◦ sintering temperature of 1700 C. At Stage V, the piston, after initial positive displacement, Materials 2021, 14, x FOR PEER REVIEW 4 of 12

Materials 2021, 14, 3812 recorded (Stages I and III). In between (Stage II), degassing of the powder mixture under4 of 11 constant temperature and force takes place. In Stage IV, a slight negative piston displacement is recorded, caused by a temperature increase from 400 °C up to the requisite sintering temperature of 1700 °C. At Stage V, the piston, after initial positive displacement, sta- stabilizesbilizes at ata constant a constant level. level. At At this this stage, stage, the the pores pores have have to to be eliminated. The lastlast stagestage (Stage(Stage VI)VI) isis thethe coolingcooling stage.stage. FurtherFurther positivepositive pistonpiston displacementdisplacement isis observedobserved duedue toto thermallythermally inducedinduced contraction.contraction.

FigureFigure 2.2. Experimental plot showing changeschanges in applied force, piston displacement andand temperaturetemperature ◦ duringduring thethe SPSSPS sinteringsintering ofof thethe TiBTiB22/20/20 wt wt % % Ti Ti55SiSi33 powderspowders (t (tmaxmax 17001700 °C/10C/10 min). min).

3.2.3.2. Relative Density TheThe powderpowder mixturemixture composition,composition, sinteringsintering parametersparameters andand relativerelative densitydensity valuesvalues of the obtained materials are shown in Table1 and compared to the TiB 2 sinter produced of the obtained◦ materials are shown in Table 1 and compared to the TiB2 sinter produced by SPS at 1700 C. The relative density of the TiB2 sinter is low in comparison to those in by SPS at 1700 °C. The relative density of the TiB2 sinter is low in comparison to those in References [4,12]. This sinter was produced using the same TiB2 powder and in the same References [4,12]. This sinter was produced using the same TiB2 powder and in the same sintering conditions as the composites and was used as the referential sample. In general, sintering conditions as the composites and was used as the referential sample. In general, the relative density of the TiB2/Ti5Si3 composites is increased in comparison to the relative the relative density of the TiB2/Ti5Si3 composites is increased in comparison to the relative density of the TiB2 sinter and increases with the sintering temperature and titanium silicide density of the TiB2 sinter and increases with the sintering temperature and titanium sili- content. This enhanced density of the produced composites comes at the expense of their cide content. This enhanced density of the produced composites comes at the expense of hardness. As the applied temperatures of sintering are lower than the melting point of their hardness. As the applied temperatures of sintering are lower than the melting point Ti5Si3, the process does not involve a liquid phase, as is the case of composites with the of Ti5Si3, the process does not involve a liquid phase,◦ as is the case of composites with the addition of TiSi2 sintered at temperatures above 1500 C; therefore, such high densities are notaddition obtained. of TiSi However,2 sintered our at temperatures composites can above operate 1500 at °C temperatures; therefore, such higher high than densities 1500 ◦ areC. not obtained. However, our composites can operate at temperatures higher than 1500 °C. Table 1. Initial phase composition of the powder mixtures and phase composition of the samples after SPS sintering, their Table 1. Initial phase composition of the powder mixtures and phase composition of the samples after SPS sintering, their hardness and relative density. hardness and relative density. Powder Mixture SPS Conditions Powder Mixture SPS Conditions Phase CompositionafterPhase Composition Sintering ρ Theoreticalρ Theoretical (%) Hardness Composition Temperature/Time Hardness Composition Temperature/Time◦ after Sintering (%) TiB2/10 wt % Ti5Si3 1600 C/35 MPa/10 min TiB2/7 wt % Ti5Si3/3 wt % TiC 94.8 1961 ± 15 HV1 ◦ TiB2/102/10 wt wt % % Ti5 TiSi35Si3 17001600C/35 °C/35 MPa/10 MPa/10 min min TiB2TiB/8 wt2/7 % wt Ti5 Si%3 /4Ti5 wtSi3 %/3 TiC wt % TiC 96.594.8 21801961± ±28 15 HV1 HV1 ◦ TiB2/20 wt % Ti5Si3 1600 C/35 MPa/10 min TiB2/16 wt % Ti5Si3/2 wt % TiC 96.9 1864 ± 40 HV1 TiB2/10 wt % Ti5Si3 1700◦ °C/35 MPa/10 min TiB2/8 wt % Ti5Si3/4 wt % TiC 96.5 2180 ± 28 HV1 TiB2/20 wt % Ti5Si3 1700 C/35 MPa/10 min TiB2/16 wt % Ti5Si3/2 wt % TiC 98.2 1953 ± 34 HV1 ◦ TiB2/20TiB wt2 % Ti5Si3 17001600C/35 °C/35 MPa/10 MPa/10 min min TiB2/16 wt TiB 2% Ti5Si3/2 wt % TiC 78.696.9 24001864± ±45 40 HV1 HV1 TiB2/20 wt % Ti5Si3 1700 °C/35 MPa/10 min TiB2/16 wt % Ti5Si3/2 wt % TiC 98.2 1953 ± 34 HV1 TiB2 17003.3. °C/35 Microstructure MPa/10 min TiB2 78.6 2400 ± 45 HV1

3.3. Microstructure

Figure 3 shows the X-ray diffraction spectra registered for the TiB2/10 Ti5Si3 powder mixture after three-step homogenization and the TiB2/10 wt % Ti5Si3 composites. The XRD analysis results confirm that the major phase present in the powder mixture after homog- Materials 2021, 14, 3812 enization and in the produced composites is TiB2 (hexagonal closest packed (hcp)). 5 of 11 The second phase identified in both powder mixtures and the TiB2/10 Ti5Si3 and TiB2/20 Ti5Si3 composites was Ti5Si3 (D88 crystal structure of the Mn5Si3). The lattice con- stants for this silicide identified in the powder sample were a = 7.465 Å and c = 5.168 Å, whilewhile in in the the composites, composites, a a = 7.4224–7.4494 Å Å and c == 5.1501–5.15065.1501–5.1506 Å werewere slightlyslightly reduced,reduced,probably probably as a result as a result of the of applied the applied pressure pressure during during sintering. sintering. The The presence presence of of boronbo- or roncarbon or carbon solid solutionssolid solutions in this in silicidethis silicide that couldthat could be formed be formed (Nowotny (Nowotny phase) phase) [13 ][13] was not wasfound, not found, and changes and changes in the in size the of size the of unit the cell unit parameters cell parameters did notdid indicatenot indicate this. this. TracesTraces of of the the TiC TiC phase phase were were detected detected in th ine thecomposites composites as a asresult a result of the of chemical the chemical reactionreaction of of titanium titanium from from the the powder powder mixtur mixturee and and carbon carbon from from the the graphite graphite components components of ofthe the sintering sintering die.die. The TiC TiC content content was was 2–4 2–4 wt wt % %of ofthe the produced produced composites. composites. Although Although aa higher higher sintering sintering temperature temperature favors favors an increase an increase in the indiffusion the diffusion rate, no rate,significant no signiﬁcant dif- ferencesdifferences were were observed observed in the in amount the amount of this ofcarbide this carbide in our composites. in our composites. Because the Because tita- the niumtitanium silicide silicide content content calculated calculated in the in compos the compositesites is lower is lower than thanassumed assumed in the in initial the initial powderpowder mixtures, mixtures, with with almost almost the the preserved preserved amount amount of tita ofnium titanium diboride, diboride, the formation the formation ofof titanium titanium carbide carbide probably probably takes takes place place at the at theexpense expense of the of loss the lossof the of titanium the titanium silicide. silicide. However,However, other other phases phases forming forming the the Ti-Si Ti-Si or Ti or-Si-B Ti-Si-B system system were were not identified, not identiﬁed, which which may may be due to the evaporation of Si (Tm = 1414 °C) ◦or the low content of these phases in the be due to the evaporation of Si (Tm = 1414 C) or the low content of these phases in the producedproduced composites, composites, below below XRD’s XRD’s detectability. detectability.

FigureFigure 3. 3.XRDXRD patterns, patterns, registered registered in BB in geometry BB geometry of the ofTiB the2 + 10 TiB wt2 +%10 Ti5 wtSi3 powder % Ti5Si 3mixturepowder after mixture ◦ ◦ homogenization,after homogenization, and the and TiB2 the/10Ti TiB5Si23/10Ti composites5Si3 composites (1600 °C and (1600 1700C ° andC) with 1700 indexedC) with peak indexed posi- peak tionspositions for the for identified the identiﬁed phases. phases. ICDD ICDD (ref. code) (ref. code)used usedfor the for identification the identiﬁcation of TiB of2 (01-075-1045), TiB2 (01-075-1045), Ti5Si3 (01-078-1429) and TiC (03-065-8805). Ti5Si3 (01-078-1429) and TiC (03-065-8805).

SEMSEM observations observations of ofthe the microstructure microstructure of the of theproduced produced composites composites indicate indicate the the presencepresence of of two two kinds kinds of grains, of grains, different different in chemical in chemical composition composition (Figures (Figures4 and 5).4 Fig- and5). 2 ureFigure 4 shows4 shows a general a general SEM SEM image image of the of themicrostructure microstructure of the of thecomposites composites TiB TiB/10 2Ti/105Si3 Ti 5Si3 (Figure(Figure 4a)4a) and and TiB TiB2/202/20 Ti5Si Ti3 5(FigureSi3 (Figure 4b) with4b) with EDS EDSmaps maps of the of elemental the elemental distribution distribution of of Ti, B and Si. Areas clearly enriched in Si are unevenly dispersed in a homogeneous matrix in terms of the content of B and Ti (Figure4, EDS maps). The surface of Si-enriched areas increases with the addition of titanium silicide. Some grain growth is observed with sintering temperature. In the SEM images of the microstructure of composites produced with a higher titanium silicide content, Si-rich areas are larger and easily recognizable as light-gray components of the microstructure of an irregular shape distributed in the dark-gray matrix. The darkest areas are pores. Figure5a shows an SEM image showing the microstructure of TiB2/10 Ti5Si3 with marked areas for EDS analysis and EDS spectra taken from them (Figure5b–d). In this magniﬁed image, some pores are evident. They most often occur within the grain boundaries, often in the vicinity of grains rich in Si, which may be a result of the evaporation of this element as the sintering temperature was ~150–250 ◦C Materials 2021, 14, x FOR PEER REVIEW 6 of 12

Ti, B and Si. Areas clearly enriched in Si are unevenly dispersed in a homogeneous matrix in terms of the content of B and Ti (Figure 4, EDS maps). The surface of Si-enriched areas increases with the addition of titanium silicide. Some grain growth is observed with sintering temperature. In the SEM images of the microstructure of composites produced with a higher titanium silicide content, Si-rich areas are larger and easily recognizable as light- gray components of the microstructure of an irregular shape distributed in the dark-gray matrix. The darkest areas are pores. Figure 5a shows an SEM image showing the micro-

Materials 2021, 14, 3812 structure of TiB2/10 Ti5Si3 with marked areas for EDS analysis and EDS spectra taken from6 of 11 them (Figure 5b–d). In this magnified image, some pores are evident. They most often occur within the grain boundaries, often in the vicinity of grains rich in Si, which may be a result of the evaporation of this element as the sintering temperature was ~150–250 °C aboveabove its its meltingmelting point. Silicon Silicon evaporation evaporation may may be be the the reason reason for for the the failure failure in achieving in achieving thethe full full densitydensity ofof ourour composites. The The loss loss of of this this element element may may cause cause differences differences in the in the Ti/SiTi/Si ratioratio in the selected areas areas of of the the prod produceduced composites, composites, as asshown shown in inFigure Figure 5 (EDS5 (EDS SpectraSpectra 11 andand 3)3)

(a) (b)

FigureFigureMaterials 4. SEM4. SEM 2021 images ,images 14, x FOR of of the PEER the microstructure micr REVIEWostructure of of the the TiB TiB2/102/10 TiTi55Si3 ((a)) and and TiB TiB22/20Ti/20Ti5Si5Si3 (3b)( bcomposites) composites produced produced by the by theSPS SPS 7 of 12

methodmethod (general (general view view with with EDS EDS maps maps showing showing thethe Si,Si, B and Ti distribution).

(a) (b)

◦ Figure 5. (a) SEM image of the microstructure of the TiB210Ti5Si3 composite (1600 C/10 min) with marked areas for EDS Figure 5. (a) SEM image of the microstructure of the TiB210Ti5Si3 composite (1600 °C/10 min) with marked areas for EDS analysis; (banalysis;–d) EDS (b spectra–d) EDS taken spectra from taken them. from them.

3.4. Hardness and Friction–Wear Properties

The average hardness values of the produced TiB2/Ti5Si3 composites are given in Ta- ble 1 and compared to the hardness measured for the TiB2 sinter produced by SPS at 1700 °C/10 min. SPS sintering of TiB2 powders at 1600 °C was not effective, and the material densified at the lower sintering temperature was not qualified for further research. Detailed SEM observation of the surfaces of the TiB2/Ti5Si3 composites after the hardness test (Figure 6a,b) indicated that the produced materials were brittle. In the indented area and in its close proximity, there were splinters and/or chippings. Cracking in the corners of the produced indents was evident and propagated intergranular. The susceptibility to chipping and radial cracking formation was increased in the composites containing 20 wt % of titanium silicide addition in comparison to those containing 10 wt % of silicide addition. Strong chipping of the surface under the indenter made the hardness measurement difficult or even impossible. The presence of chipping and spallation of the surface of the specimens near the imprints made it impossible to calculate KIC. In general, there are no data that can be found in the field of the microstructure and properties of TiB2 composites prepared by the SPS method with Ti5Si3 as a sintering aid; however, this phase is often identified in composites produced by the sintering of TiB2 powder with TiSi2 [9,14] or MoSi2 [15] sintering additives. The Ti5Si3 phase occurs in these composites as an unwanted product of the reactions between TiB2 and TiSi2, occurring during spark plasma

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3.4. Hardness and Friction–Wear Properties

The average hardness values of the produced TiB2/Ti5Si3 composites are given in Table1 and compared to the hardness measured for the TiB 2 sinter produced by SPS ◦ ◦ at 1700 C/10 min. SPS sintering of TiB2 powders at 1600 C was not effective, and the material densified at the lower sintering temperature was not qualified for further research. Detailed SEM observation of the surfaces of the TiB2/Ti5Si3 composites after the hardness test (Figure6a,b) indicated that the produced materials were brittle. In the indented area and in its close proximity, there were splinters and/or chippings. Cracking in the corners of the produced indents was evident and propagated intergranular. The susceptibility to chipping and radial cracking formation was increased in the composites containing 20 wt % of titanium silicide addition in comparison to those containing 10 wt % of silicide addition. Strong chipping of the surface under the indenter made the hardness measurement difficult or even impossible. The presence of chipping and spallation of the surface of the specimens near the imprints made it impossible to calculate KIC. In general, there are no data that can be found in the field of the microstructure and properties of TiB2 composites prepared by the SPS method with Ti5Si3 as a sintering aid; however, this Materials 2021, 14, x FOR PEER REVIEW 8 of 12 phase is often identified in composites produced by the sintering of TiB2 powder with TiSi2 [9,14] or MoSi2 [15] sintering additives. The Ti5Si3 phase occurs in these composites as an unwanted product of the reactions between TiB2 and TiSi2, occurring during spark sinteringplasma sintering at 1400–1650 at 1400–1650 °C for 10◦C min for 10but min also but in also composites in composites sintered sintered at a relatively at a relatively low temperaturelow temperature of 1200 of °C 1200 but◦ Cfor but a longer for a longertime duration time duration (i.e., 30 (i.e.,min). 30 In min). a material In a materialcontain- ingcontaining 5 wt % TiSi 5 wt2, %the TiSi Ti52Si, the3 phase Ti5Si was3 phase observed was observed near the interfaces near the interfaces in between in betweenthe TiB2 and the TiSiTiB22 andgrains TiSi as2 grainsa layer asof a50–100 layer ofnm 50–100 in thickness. nm in thickness. Unfortunately, Unfortunately, despite the despite fact that the this fact phasethat this was phase identified was identified by XRD, by its XRD, content its content was not was estimated, not estimated, so its soinfluence its influence on the on me- the chanicalmechanical properties properties of produced of produced material material is not is not known. known. In Inthe the absence absence of ofinformation, information, it remainsit remains to compare to compare the measured the measured hardness hardness values values to the tocalculated the calculated values on values the basis on the of thebasis rule of of the mixtures. rule of mixtures. Assuming Assuming TiB2 hardness TiB2 hardnessof 2400 HV1 of 2400 (Table HV1 1) (Tableand Ti15)Si and3 hardness Ti 5Si3 ofhardness 986 HV1, of 986according HV1, according to [16], the to calculated [16], the calculated hardness hardness values are values ~2258 are HV1 ~2258 for HV1TiB2/10 for wtTiB %2/ 10Ti5 wtSi3 %compositesTi5Si3 composites and 2117 and HV1-for 2117 HV1-for TiB2/20 TiBwt2 /20% Ti wt5Si3 %, which Ti5Si3, correspond which correspond to the measuredto the measured values values (Table (Table1). The1). decrease The decrease in the in hardness the hardness of the of produc the produceded composites composites with Tiwith5Si3 Ticontent5Si3 content is not issurprising, not surprising, as similar as similar degradation degradation of the ofproperties the properties of the ofTiB the2-based TiB2- compositesbased composites is observed is observed with the with addition the addition of other of phases other phases beyond beyond 5 wt % 5 of wt similar % of similar hard- nesshardness to that to of that TiB of2, TiBsuch2, suchas MoSi as MoSi2 [15].2 [15].

(a) (b)

◦ Figure 6. (a) Light microscopy imageimage ofof thethe surfacesurface ofof thethe indentedindented area:area: TiBTiB22/10/10 Ti 5SiSi33 compositecomposite sintered sintered at at 1600 1600 °CC( (bb);); SEM image of the surface of the indented area of the TiB2/20 Ti5Si3 composite sintered at 1600 °C.◦ SEM image of the surface of the indented area of the TiB2/20 Ti5Si3 composite sintered at 1600 C.

TheThe friction–wear friction–wear properties properties of of the the TiB TiB2/Ti2/Ti5Si35 Sicomposites3 composites were were examined examined in the in ball- the on-discball-on-disc test without test without the use the of use lubricants. of lubricants. In our In experiments, our experiments, the resistance the resistance to wear to wear of the of tested samples was determined against a WC ball, as was already been applied in our previous studies for TiB2 sinters [17]. Figure 7 shows the CoF and wear rate plots of the composite samples tested in the dry sliding test (ball-on-disc test). The addition of Ti5Si3 significantly reduces the wear rate of the produced composites. With an increased content of Ti5Si3, a decrease in the coefficient of friction values is noted at a low number of cycles (N < 2000) but does not influence the average value of the COF, as shown in Figure 8.

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the tested samples was determined against a WC ball, as was already been applied in our previous studies for TiB2 sinters [17]. Figure7 shows the CoF and wear rate plots of the composite samples tested in the dry sliding test (ball-on-disc test). The addition of Ti5Si3 significantly reduces the wear rate of the produced composites. With an increased content Materials 2021, 14, x FOR PEER REVIEWof Ti Si , a decrease in the coefficient of friction values is noted at a low number9 of of 12 cycles 5 3 Materials 2021, 14, x FOR PEER REVIEW(N < 2000) but does not influence the average value of the COF, as shown9 of in12 Figure8.

(a()a ) (b) (b) Figure 7. Experimental plots of the CoF and the wear rate of the SPS sinters tested in the dry sliding test with WC as a FigureFigure 7. Experimental 7. Experimental plots plots of of the the CoFCoF andand the wear wear rate rate of of the the SP SPSS sinters sinters tested tested in the in thedry drysliding sliding test with test withWC as WC a as a counterpart: (a) TiB2/10Ti5Si3 1600 °C; (b) TiB2/20Ti5Si3 1700 °C. counterpart: (a) TiB2/10Ti5Si3 1600 °C;◦ (b) TiB2/20Ti5Si3 1700 °C. ◦ counterpart: (a) TiB2/10Ti5Si3 1600 C; (b) TiB2/20Ti5Si3 1700 C.

Figure 8. CoF values and volumetric wear indexes (Wv) of the TiB2/Ti5Si3 composites determined in Figure 8. CoF values and volumetric wear indexes (Wv) of the TiB2/Ti5Si3 composites determined in friction contact with WC balls loaded at 5 N. friction contact with WC balls loaded at 5 N. Figure 8. CoF values and volumetric wear indexes (Wv) of the TiB2/Ti5Si3 composites determined in frictionThe specificcontact withwear WC rate balls values loaded Wv areat 5 relatively N. low and similar for all produced com- The speciﬁc wear rate values Wv are relatively low and similar for all produced posites. In comparison to the value for the referential TiB2 sinter tested with the WC ball incomposites. the sameThe specificball-on-disc In comparison wear test rate parameters, values to the Wv valueit was are reducedrelatively for the by referential low ~28 and%. To similar understand TiB2 sinterfor all the produced tested cause with com- the WC ofballposites. the in significant the In samecomparison increase ball-on-disc in to the the wear value test resistance parameters,for the referentialwith Ti it5Si was3 content, TiB reduced2 sinter the weartested by tracks ~28 with %. were the To WC understand ball subjectedthein the cause same to ofdetailed ball-on-disc the signiﬁcant microstructural test parameters, increase observatio in it thewasns with wear reduced the resistance use by of ~28 a scanning %. with To understand Tielectron5Si3 content, mi- the cause the wear croscope (Figure 9) and accompanying analyses of the chemical composition by the EDS tracksof the weresignificant subjected increase to detailedin the wear microstructural resistance with observations Ti5Si3 content, with the wear the use tracks of awere scanning method.electronsubjected microscope to detailed (Figuremicrostructural9) and accompanying observations with analyses the use of of the a scanning chemical electron composition mi- by thecroscope EDS method. (Figure 9) and accompanying analyses of the chemical composition by the EDS method.In the areas of the friction track, numerous grooves were observed, which are typical signs of abrasive wear. EDS maps registered from worn areas indicated the presence of oxygen in the worn area, as well as traces of tungsten and carbon. The presence of boron oxides (and hydroxides), as well as titanium and silicon oxides, was the result of the tribo-oxidation of Ti, B and Si coming from the tested composite samples. Traces of W and C, which were also detected by EDS, come from the WC ball. The distribution of W (Figure 10) also indicates the presence of the tribo-oxidation products of this element. Tungsten and titanium suboxides belong to the group of Magnéli phases with lubricating

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properties [18,19]. Hydrogenated boron oxides have similar lubricating properties. Their presence in the friction contact area is favorable; however, they do not signiﬁcantly affect the CoF value (Figure8). Clearly, tribo-oxidation takes place at the expense of the weight Materials 2021, 14, x FOR PEER REVIEWloss of both materials in frictional contact, but the formation of oxides compensates10 of for 12 this loss, which, in turn, has a positive effect on the speciﬁc wear rate values.

(a) (b)

Materials 2021, 14, x FOR PEER REVIEWFigure 9. SEM images of the surface of the TiB2/20 wt % Ti5Si3 composite after the ball-on disc test:11 of 12 Figure 9. SEM images of the surface of the TiB /20 wt % Ti Si composite after the ball-on disc test: (a) general view (low-vacuum SE image); (b) magnified2 SEM image5 3 of the wear track area. (a) general view (low-vacuum SE image); (b) magniﬁed SEM image of the wear track area. In the areas of the friction track, numerous grooves were observed, which are typical signs of abrasive wear. EDS maps registered from worn areas indicated the presence of oxygen in the worn area, as well as traces of tungsten and carbon. The presence of boron oxides (and hydroxides), as well as titanium and silicon oxides, was the result of the tribo- oxidation of Ti, B and Si coming from the tested composite samples. Traces of W and C, which were also detected by EDS, come from the WC ball. The distribution of W (Figure 10) also indicates the presence of the tribo-oxidation products of this element. Tungsten and titanium suboxides belong to the group of Magnéli phases with lubricating properties [18,19]. Hydrogenated boron oxides have similar lubricating properties. Their presence in the friction contact area is favorable; however, they do not significantly affect the CoF value (Figure 8). Clearly, tribo-oxidation takes place at the expense of the weight loss of both materials in frictional contact, but the formation of oxides compensates for this loss, which, in turn, has a positive effect on the specific wear rate values.

FigureFigure 10. 10. EDSEDS elemental elemental maps maps of of Ti, Ti, O, W and C register registereded from the area presentedpresented in Figure9 9b.b.

4. Conclusions

The effects of sintering temperature and Ti5Si3 content on the microstructure of TiB2/Ti5Si3 composites, their density, hardness and friction–wear properties were investigated and discussed. Based on the experimental results and analysis, the following major conclusions were reached: • Spark plasma sintering is an effective and fast method for the densification of TiB2/Ti5Si3 powders. The addition of 10–20 wt % of Ti5Si3 is beneficial for the density of TiB2/Ti5Si3 composites. The relative density of the produced composites is up to

98.4% of the theoretical composites. • SPS sintering of the TiB2/Ti5Si3 initial powders at 1600 °C and 1700 °C results in the formation of TiC due to the effect of carbon diffusion from the graphite components of the sintering die. • The hardness of the produced composites decreases with Ti5Si3 content, but their resistance to wear in friction contact with WC increases with it, and for composites containing 20% of this additive, it increases by almost 30%. • The main mechanism of wear is abrasion. The presence of titanium, tungsten and boron oxides at worn areas indicate the tribo-oxidation reactions of these elements. • The COF values of TiB2/Ti5Si3 composites in friction contact with WC were in the range of 0.54–0.61, similar to the value of the TiB2 sinter.

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4. Conclusions

• Spark plasma sintering is an effective and fast method for the densification of TiB2/Ti5Si3 powders. The addition of 10–20 wt % of Ti5Si3 is beneﬁcial for the density of TiB2/Ti5Si3 composites. The relative density of the produced composites is up to 98.4% of the theoretical composites. ◦ ◦ • SPS sintering of the TiB2/Ti5Si3 initial powders at 1600 C and 1700 C results in the formation of TiC due to the effect of carbon diffusion from the graphite components of the sintering die. • The hardness of the produced composites decreases with Ti5Si3 content, but their resistance to wear in friction contact with WC increases with it, and for composites containing 20% of this additive, it increases by almost 30%. • The main mechanism of wear is abrasion. The presence of titanium, tungsten and boron oxides at worn areas indicate the tribo-oxidation reactions of these elements. • The COF values of TiB2/Ti5Si3 composites in friction contact with WC were in the range of 0.54–0.61, similar to the value of the TiB2 sinter.

Author Contributions: Conceptualization, A.T.; methodology, A.T. and M.P.; validation, K.B., P.H. and I.S.; formal analysis, A.T.; investigation, I.S. and P.H.; resources, A.T.; data curation, K.B. writing— original draft preparation, A.T.; writing—review and editing, A.T.; project administration, A.T.; funding acquisition, A.T. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: This work was financially supported by Pedagogical University in Krakow, Poland, within the research project WPBU/2020/05/00195. Conflicts of Interest: The authors declare no conflict of interest.

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