Popov O. Reactive sintering and structure design of boron-containing ceramic materials

A model for ceramic composite fracture energy and toughness estimation has been developed as a theoretical basement for structure optimization. The model allows to estimate the effects of fracture delaying in interfacial border before the phase with higher Young’s modulus, and the possibility of crack front bending between stoppers before the final material destruction. Henceforth, two types of composite structure were shown to be optimal for ceramic mechanical characteristics improvement: - fine matrix with rough (more than 30µm) inclusions of higher Young’s modulus; - high-E matrix with 10 – 15vol.% of submicron voids or soft inclusions of graphite or graphite-like boron nitride. The purpose of further investigation of refractory phase formation mechanisms was the development of reaction sintering approach to such structures creation. To introduce submicron graphite inclusions into high-E boride-based matrix, a sequence of ceramic samples was made by means of titanium and boron carbides powder mixtures hot pressing at 1800 – 1950℃, 30MPa for 16 min. X-ray diffractometry of sintered materials showed that high-temperature annealing of the charges provoked reaction, resulting titanium diboride and graphite appearing. The investigation of mechanical characteristics of ТіВ2-ТіС-С, ТіВ2-В4С-С, and ТіВ2-С (depending on the initial powder composition) ceramics allowed concluding that both microhardness and fracture toughness of sintered materials depended on graphite content mostly. While hardness decreased monotonically with soft phase fraction increasing, fracture toughness had clear maximum at ~12vol% of graphite. The obtained K1C on soft phase content dependence correlated with model estimations. Mechanical characteristics of created materials amounted: К1С = 10±0.4MPa‧m1/2, HV = 24±1GPa. Experimental investigation of TiC – B4C reaction and corresponding heteromodulus composite structure formation mechanisms is also presented. The reaction between titanium and boron carbides is shown to begin at 1100 – 1200℃ with boron atoms in titanium carbide grains accumulation near interphase contact areas and consequent titanium diboride nanoplates along TiC (111) layers appearing. Considerable reaction acceleration at 1600 – 1800℃ is caused by boron from B4C sublimation and its transfer to all free TiC surfaces. The TiB2 flakes growth inside titanium carbide crystals causes tensile stress increasing and submicron TiC particles breaking from parental grain surface. Thus sublimated boron can condense on fresh carbide plains and the reaction progresses avoiding the necessity of boron through titanium diboride solid phase diffusion. Carbon captured between new formed TiB2 nuclei, segregates as submicron platelets in TiC-TiB2 interface creating the optimal heteromodulus composite structure. Temperature dependences of titanium and aluminium transformation degree into corresponding refractory phases while annealing Al-B2O3, Ti-B2O3 and Ti-Al-B2O3 mixtures at 900 – 1400℃ showed that aluminium to titanium and boron oxide powders addition improves both titanium diboride and alumina formation. Similar investigation of Al-Cr2O3-B2O3 system revealed the third component (boron oxide) addition results reaction excitation temperature decreasing by 300℃. The main feature of three-component system phase composition evolution is fusible TiAl3 or CrBO3 intermediate compounds appearance on titanium or chromium oxide grain surface respectively. A mechanism of three-component system reaction is shown to be precursor dissolution in intermediate compound melt, followed by refractory TiB2, CrB and Al2O3 products from the liquid nucleation. As refractory phase formation in Ti-Al-B2O3 can be completed at 1400℃ for tens of seconds we decided to investigate the possibility of bulk material sintering in the system. The reaction between titanium, boron oxide and aluminium is shown to occur with 44% volume reduction, thus the compact composite structure is forming through three consequent stages. The first stage is porosity with aluminium and boron oxide melts filling with pressure application. Further charge heating leads to reaction completion and secondary porosity formation. Titanium diboride and alumina construct a hard matrix which can be deformed if the external pressure surpasses their yield strength. The latter occurs at 1600℃ and 30MPa during the third stage of sample densification when the secondary porosity disappears because of alumina grains plastic flow and nonporous material with microhardness of 24GPa and fracture toughness of 8MPa‧m1/2 forms. To apply reactive sintering method to manufacturing of ceramics with earlier estimated structure of fine matrix with rough high-E inclusions, materials basing on Ti-Al-B2O3-TiB¬2, Cr2O3-Al-TiB2, and B2O3-Al-C-TiB2 precursors were produced and investigated. Composite structure of the sample made with hot pressing of B2O3-Al-C-TiB2 charge at 1900℃ and 30MPa for 8min. presented a combination of fi