Hot Pressing of Boron Carbide Based Ceramic Composites

August 2020, Vol. 20, No. 3 MANUFACTURING TECHNOLOGY ISSN 1213–2489

Pavol Švec, Zuzana Gábrišová, Alena Brusilová Institute of Technologies and Materials, Faculty of Mechanical Engineering, Slovak University of Technology in Bratislava. Námestie slobody 17, 812 31 Bratislava. Slovak Republic. E-mail: [email protected], [email protected], [email protected]

Ceramic composite materials based on boron carbide were hot pressed utilising in situ reaction of boron carbide powder with 40 wt.% of titanium dioxide sintering additive. The samples were prepared at sintering temperature of 1850 °C, pressure of 35 MPa, and time of 60 min in vacuum atmosphere of about 20 Pa. Optimisation of sintering regime concentrating on both heating stage of sintering and application of pressure enabled to prepare fully dense ceramic composite materials composed of boron carbide matrix with 29.5 vol.% of titanium diboride secondary phase. The ceramic composite reached average density of 99.31 %, hardness of 29.8 GPa, and fracture toughness of 6.9 MPa.m1/2.

Keywords: boron carbide, ceramic composite, sintering, microstructure

strength, hardness and fracture toughness. However, Introduction the fracture toughness of B4C based ceramics must be Boron carbide (B4C) based ceramics and ceramic improved for successful application of this ceramic composites with B4C matrix have been intensively material [3, 7-10]. studied materials because of their extraordinary prop- Both problems of preparation of B4C based ceram- erties that stem from their microstructure. Boron car- ics decreasing of sintering temperature and increasing bide is according its mainly covalent bonds extremely of fracture toughness of samples can be solved by sin- hard, corrosion resistant and wear resistant material at tering of ceramic composite systems. Selection of suit- both room and elevated temperatures. Low density, able composite systems enables creation of new high Young modulus and high absorption cross sec- phases in situ during sintering process of B4C powder tion for neutrons are other important properties of with proper sintering additives. Sintering additives im- B4C. One important advantage in comparison with prove densification and influence microstructure and many ceramic materials is good electric conductivity, mechanical properties of boron carbide based compo- which enables to form products from B4C by electrical site materials because of creation of secondary phases. discharge machining [1-4]. Based on its properties B4C The secondary phases are significant mainly for in- ceramic and ceramic composites are promising mate- creasing of fracture toughness, which is critical for rial for production parts working in demanding work- B4C based ceramic materials because it reaches values 1/2 ing conditions. Extraordinary hardness predestines only from 2.2 to 3.7 MPa.m . Hardness of B4C based B4C for applications requiring high abrasive and ero- ceramic composite usually decreases, because phases sive wear resistance such as sand-blast nozzles and wa- with lower hardness create compared to B4C. Using of ter jet nozzles. Small density and high modulus of elas- fine-grained powders and application of pressure, ticity make B4C based ceramics a perspective material which can be applied at hot pressing, during sintering for light armour such as bullet-proof vests and aircraft process enables decreasing of sintering temperature applications. High active cross section of B4C for neu- too. [1, 4, 11-14]. trons absorption can be utilised in nuclear technique Several metals (Al, Hf, Ti), non-metals (B, C) ox- [3-6]. ides (Al2O3, HfO2, TiO2, ZrO2), and non-oxides The potential of B4C is decreased by both difficult (Si3N4, TiC, W2B5) are appropriate sintering additives sinterability and low fracture toughness of this ceramic for in situ sintering of B4C based ceramic composites material. Difficult sinterability of B4C stems from the [8-10, 12, 13]. At the sintering of B4C powder with ti- low self-diffusion coefficients in this ceramic system. tanium dioxide additive (TiO2), the oxidic additive re- Relatively high temperatures, above 2300 °C, are nec- acts with B4C and form the titanium diboride (TiB2), essary to achieve compacts with the density near to the which become a part of phase composition of created theoretical density of 2.52 g.cm-3. However, the sinter- composite material. The process of in situ boride cre- ing of B4C ceramics at temperatures above 2000 °C is ation significantly enhances of B4C sinterability [9, 12, accompanied with particle coarsening in ceramic com- 14-16]. From thermodynamic presumption stems that pacts which decrease mechanical properties such as B4C- TiB2 ceramic composite material is created in

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consequence of in situ reaction between B4C and TiO2 pressed in graphite die with floating matrix of cylindri- initial powders, but the volatile species such as CO and cal shape with a diameter of 12 mm. Several hot press- CO2 are a part of in situ reaction, too. The final phase ing regimes with different heating stages of sintering composition of sintered samples changes with portion processes were applied. However, the final process pa- of TiO2 in the initial powder mixture. The portion of rameters such as sintering temperature of 1850 °C, volatile species grows with increased ratio of TiO2 in pressure of 35 MPa, sintering time of 60 min and a the initial B4C-TiO2 powder mixture. Formation of vacuum atmosphere about 20 Pa were the same for all volatile components complicates sample preparation. regimes, because they allowed optimal combination of Increased care is necessary to give to their removing mechanical properties according the result in work because removing of volatile species affect the densi- [17]. fication of ceramic and so the rest-porosity of sintered The surfaces of bulk samples were observed using samples [4, 8, 10, 17]. stereomicroscope Zeiss Technival 2. The densities of Microstructure and properties of B4C-TiB2 ce- hot pressed samples were measured using Archimedes -3 ramic composite materials depend mainly on volume method. Theoretical density of B4C-TiB2 (3.11 g.cm ) portion of created phases, densification, and grain ceramic composite was calculated based on both the- -3 size. The proper densification of every ceramic com- oretical densities of B4C (2.52 g.cm ) and TiB2 (4.52 posite with the relative density near full density is a g.cm-3) phases and their volume portions (70.5 vol.% necessary condition for advanced properties of ce- B4C 29.5 vol.% TiB2) measured using the image anal- ramic samples. The fracture toughness of B4C-TiB2 ysis for samples hot pressed at optimal sintering re- composite increases with portion of TiB2 phase in gime. Relative densities of B4C-TiB2 ceramic compo- consequence of better fracture of TiB2 phase com- sites hot pressed at different sintering regimes were pared to B4C phase. The hardness decreases with por- calculated by comparison of their densities with the -3 tion of TiB2 phase because of lower hardness of TiB2 theoretical density value of 3.11 g.cm (100 %). The phase compared to B4C phase [4, 9, 15, 18]. microstructures were studied on cross sections of ce- Hot pressing process is one of preferred processes ramic composite samples using light microscopy with at the densification of B4C based ceramics, because Axiovert 40 MAT microscope and using electron mi- simultaneous application of temperature and pressure croscopy with JEOL JSM-IT300 scanning electron enables significant decreasing of sintering temperature microscope. The phase analysis was done using X ray compared to pressureless sintering [4, 10, 13, 15, 19]. diffraction method with Philips PW 1710 diffractom- Proper outgassing of B4C based ceramic composite eter. Volume portions of identified phases were meas- materials is important at hot pressing process with in ured using image analysis. The hardness and fracture situ reaction. Although hot pressing of B4C based ce- toughness were measured by indentation method us- ramics has been studied in several works and essential ing Vickers indenter Buehler IndentaMet 1100. information about the final sintering temperature have been published, the information about heating stage Results and discussion of sintering process and application of pressure is less Sintering regime of boron carbide based ceramic presented. An optimal sintering regime is necessary composite material was optimised to enable removing for both proper removing of volatile species during of volatile species created by in situ reaction and mi- sintering and preparation of dense compacts with en- crostructure formation during hot pressing of powder hanced properties and it was the aim of this study with mixtures with the initial composition of B4C with 40 concentration of heating stage of hot pressing. wt.% TiO sintering additives. Although different sin- Used materials and evaluation methods tering regimes were applied, the same final sintering temperature of 1850 °C, pressure of 35 MPa, time Hot pressing process was chosen for fabrication of pressing time of 60 min in vacuum atmosphere about B4C-TiB2 ceramic composite materials from B4C and 20 Pa were applied for all composite samples. TiO2 initial powders with the purity of 99 % and a par- Optimization of sintering regime during hot pressing ticle size from 2 to 3 µm. Initial powder mixture con- of B4C based ceramic composites tained 40 wt.% of TiO2 sintering additive, because this composition was optimised in work [17]. The B4C and The optimization of sintering regime was focused TiO2 initial powder mixture with 1 wt.% of binding on both heating stage of sintering process and appli- wax was milled in Teflon container with B4C mill balls cation of pressure considering in situ reaction. The op- and isobutyl alcohol lubricant for 4 hours in the hori- timal sintering regime should enable to remove the zontal mill. After drying of initial powder mixture, pre- volatile species which accompanied in situ reaction cursors of cylindrical shape with a diameter of 12 mm and prepare the ceramic composites without the rest were prepared by die pressing in simple steel tool with porosity. The in situ reaction kinetic during hot press- floating die. The precursors were consequently hot ing of B4C and TiO2 initial powder mixtures was identified by measuring of vacuum level in chamber of hot

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press using vacuum gauge because the in situ reaction stage of in situ reaction at the temperature of 1570 °C was accompanied by decrease of vacuum level. The caused creation of several cracks and large pores. W5Rh-W26Rh thermocouple was used for the meas- These defects can be seen on the base of the cylindri- uring of sintering temperature. The densification pro- cal surface of B4C based ceramic composite compact gress during sintering of ceramic composite samples in fig 3. It could be considered that the porosity of was observed according to motion of punch of die ceramic composite fulfilled the function of canals for tool during hot pressing process and it allowed to cal- removing of volatile species (CO and CO2) created by culate the densification speed in different sintering in situ reaction during hot pressing process. The phases. cracks were the consequence of premature application of pressure. The density values of these samples were 2000 50 91.18 ± 1.22 % and are presented in fig. 6. Several sur- 1800 T 45 p face defects caused relatively large scattering of meas- 1600 40 ured density values as can be seen in fig. 6. As these 1400 35 values are not enough for reaching of advanced me- 1200 30 chanical properties of sintered ceramic composite 1000 25 compacts, mechanical properties of samples were not 800 20 T [°C] p [MPa] measured. 600 15 400 10 2000 200 T 200 5 1800 180 vac 0 0 1600 160 0 30 60 90 120 150 180 210 240 1400 140 t [min] 1200 120 Fig. 1 Progress of sintering temperature (T) and pressure (p) 1000 100 800 80 during initial sintering regime T [°C] vac [Pa] 600 60 The initial sintering regime applied during the hot 400 40 pressing of ceramic composite samples is demon- 200 20 0 0 strated in fig. 1. Hot pressing in accordance with this 0 102030405060708090100110120 initial sintering regime (fig. 1) was characterised by t [min] progressive application of sintering pressure after reaching the temperature of 1500 °C. In this stage of Fig. 2 Progress of sintering temperature (T) and vacuum level hot pressing, the vacuum level rapidly decreased, (vac) during hot pressing of B4C based ceramic composites which indicated intensive in situ reaction between B4C and TiO2 initial powder components. Progress of sin- Based on the results achieved during the hot press- tering temperature and vacuum level during hot pressing with the initial sintering regime in fig. 1, the heating ing of B4C based ceramic composites is depicted in fig. stage of hot pressing process was several times modi- 2. According to the vacuum level in fig. 2, volatile spe- fied. Significant progress in densification of hot cies significantly created in the temperature interval pressed B4C based ceramic composite materials was from 1500 to 1850 °C. Creation of relative high por- achieved by adding of a dwell at the temperature of tion of volatile species, probably CO and CO2, was 1570 °C for time of 60 min during the heating stage of confirmed by measuring of weight loss of hot pressed a modified sintering regime. The dwell was added to samples, which was about 30 wt.%. The maximal sin- prolongate the stage during removal of created volatile tering pressure of 35 MPa was applied at the temper- species during the densification of sample when the ature about 1570 °C. The densification speed reached material has still adequate open porosity. The modi- the maximal value of 2.5 mm.min-1 shortly after appli- fied sintering regime had a positive effect on decreas- cation of full pressure of 35 MPa at the temperature ing of cracks on surface of samples, but the densifica- of 1570 °C. Later, densification speed decreased and tion of samples was insufficient. The density reached at the end of the dwell at the final sintering tempera- the value of 96.24 ± 0.32 %. When comparing the ture of 1850 °C for 60 min it stabilised at the value of densities achieved at modified sintering regime with 0.01 mm.min-1. At this stage of sintering process, the the initial regime (see fig. 6), significant increase of vacuum level in chamber of hot press chamber density with narrowed interval was achieved. How- reached the minimal value about 20 Pa. ever, the differences in the density values across the It was not possible to obtain ceramic composite samples were observed on cross section of samples compacts without surface defects using the initial sin- hot pressed with this modified sintering regime. Alt- tering regime during hot pressing of ceramic compo- hough the surfaces of samples were properly densi- site samples. Application of pressure at the beginning fied, the centres of samples proved relatively large

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amount of rest porosity. The rest porosity in the cen- %. The densities achieved at the optimal sintering re- tres of samples was caused by premature densification gime are compared with densities measured at initial of surface areas of samples, before the volatile species and modified sintering regimes in fig. 6. The average could be removed from the bulk volume. density at the optimal regime (99.31 %) increased significantly compared to modified regime (96.24 %), but variation of measured densities improved only slightly. Hot pressed bulk compacts were without any surface defects, as can be seen in fig. 5, where the cylindrical ceramic composite sample with a diameter of 12 mm and a height of 12 mm is documented.

Fig. 3 Surface cracks and pores created in B4C based ceramic composite hot pressed using initial sintering regime

2000 50

1800 T 45 p 1600 40 1400 35 Fig. 5 B4C based ceramic composite without surface defects 1200 30 hot pressed using optimal sintering regime 1000 25 800 20 T [°C]

p [MPa] p 99 600 15 98 400 10 97 200 5 96 0 0 0 30 60 90 120 150 180 210 240 95 t [min] 94 93 Fig. 4 Progress of sintering temperature (T) and pressure (p) [%] Density during optimal sintering regime 92 91

Based on the results achieved during hot pressing 90 using several modified sintering regimes, it was found that the full sintering pressure should be applied at Initial Modified Optimal Sintering regime higher temperature compared to previous regimes. So, the hot pressing process was modified as can be seen Fig. 6 Comparison of densities at different sintering regimes in fig. 4 and this regime could be labelled as the optimal sintering regime. During the heating stage of the Microstructure of B4C-TiB2 ceramic composites optimal sintering regime a dwell at the temperature of The microstructures of B4C based ceramic compo- 400 °C for time of 15 min was added to improve the sites hot pressed at the same final sintering tempera- removing of binding wax added at cold pressing of ture of 1850 °C, the same sintering time of 60 min, precursors. Progressive application of pressure started and the same pressure of 35 MPa, but at applying of only at the maximal sintering temperature of 1850 °C. different sintering regimes are documented in fig. 7 The optimal sintering regime allowed both the elimi- (modified sintering regime) and 8 (optimal sintering nation of cracks and proper densification of ceramic regime). The microstructures of both presented sam- composite samples with the density of 99.31 ± 0.26 ples consist of two phases which identification was

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confirmed by XRD analysis. Dark areas in both mi- ondary phase. Fully dense microstructure was the con- crographs represent boron carbide (B4C) phases and sequence of advanced mechanical properties and the light areas represent titanium diboride (TiB2) phases. B4C-TiB2 ceramic composite reached the average Without these phases, the sample in fig. 7, prepared hardness value of 29.8 GPa and the average fracture using modified sintering regime, and with the average toughness of 6.9 MPa.m1/2. These values can be com- density of 96.24 % shows relatively large portion of pared with the results in works [2, 4, 8, 10, 11], where rest porosity. Because of relatively low relative density similar mechanical properties were measured. values of hot pressed B4C-TiB2 ceramic composite depicted in fig. 7, mechanical properties of the compo- Conclusions site were not measured. Boron carbide based ceramic composite materials were hot pressed utilising in situ reaction of B4C powder with 40 wt.% of TiO2 sintering additive. The in situ reaction during hot pressing of B4C and TiO2 initial powder mixture resulted in creation of ceramic composite composed of B4C matrix reinforced with TiB2 secondary phase. In situ sintering of the composite was accompanied with evolution of volatile species, which must be properly removed during hot pressing process. Based on the measurement of vacuum level during hot pressing process intensive volatile species were produced in the temperature interval from 1500 to 1850 °C. Creation of volatile species by in situ reaction caused large problems during hot pressing process. Cracks on the sample surfaces and significant porosity Fig. 7 Microstructure of B4C-TiB2 ceramic composite hot were observed at applying of pressure in sintering tem- pressed using modified sintering regime perature interval from 1500 to 1570 °C. The porosity acted as canals for removing of volatile species. Prolongated dwell of 60 min at temperature of 1570 °C decreased cracks amount on the surfaces of samples, but the densification of samples was insufficient. The samples reached the density value of 96.24 ± 0.32 %. The porosity in the centres of samples was caused by premature densification of surface areas of samples, before the volatile species could be removed from the bulk volume. The optimal sintering regime enabled to prepare fully dense B4C-TiB2 ceramic composite materials. Firstly, the precursors were heated to the sintering temperature of 1850 °C, and secondly, pressure of 35 MPa was progressively applied. This sequence was necessary for achieving of ceramic composite com-

pacts without any surface defects. The ceramic com- Fig. 8 Microstructure of B4C-TiB2 ceramic composite hot posite hot pressed at optimal sintering regime were

pressed using optimal sintering regime composed of B4C matrix with 29.5 vol.% TiB2 secondary phase. The composites reached the density of Applying of the optimal sintering regime during 99.31 ± 0.26 %, average hardness value of 29.8 GPa, hot pressing process of B4C-TiB2 ceramic composites and fracture toughness of 6.9 MPa.m1/2. enabled to prepare ceramic composite samples with the average density of 99.31 %. The microstructure of Acknowledgements this B4C-TiB2 ceramic composite without visible rest porosity is depicted in fig. 8. The portions of in situ This work was supported by the Scientific Grant created phases in fig. 8 were measured using the image Agency of the Ministry of Education, Science, Re- analysis. According the image analyse the microstruc- search and Sport of the Slovak Republic under the ture of ceramic composite in fig. 8 consists of 70.5 VEGA 1/0298/18 contract. The work was sup- vol.% B4C matrix reinforced with 29.5 vol.% TiB2 sec- ported by UVP STU Bratislava the ITMS 26240220084 project.

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