Preparation Of Si₃N₄/TiC-based Ceramic Tools By SPS And Their Self-Repairing And Self-Lubricating Properties

To solve the problem that ceramic tools are sensitive to cracks and have high friction coefficient under dry cutting conditions,this paper selects Si₃N₄/TiC-based ceramic tools,and by optimizing the composition design,adds ZrSi₂ crack repairing agent and h-BN@SiO₂ coated solid lubricant into the Si₃N₄/TiC ceramic tool material matrix,and develops a new type of Si₃N₄/TiC/ZrSi₂/h-BN@SiO₂ ceramic tool,which makes the ceramic tools have both crack repairing ability and self-lubricating ability.Si₃N₄/TiC and Si₃N₄/TiC/ZrSi₂/h-BN@SiO₂ ceramic tool materials with ZrSi₂crack repair agent and h-BN@SiO₂ solid lubricant were prepared by spark plasma sintering.The research shows that when the sintering temperature is 1700?,the holding time is 10 min,and the holding time is set at 1600?for 15 min,the Si₃N₄/TiC-based ceramic tool material has high mechanical properties,and its bending strength,fracture toughness and Vickers hardness are 959 MPa,8.61 MPa·m^1/2 and15.21 GPa,respectively.The effects of ZrSi₂ and h-BN@SiO₂ on the microstructure and mechanical properties of Si₃N₄/TiC ceramic tool materials were studied.The addition of ZrSi₂promotes the transformation from?-Si₃N₄ to?-Si₃N₄.Adding h-BN@SiO₂ can avoid the degradation of mechanical properties of ceramic cutting tools by directly adding h-BN.h-BN is uniformly distributed in ceramic tool materials due to the binding effect of outer shell SiO₂,and the microstructure shows that some h-BN forms an intragranular structure in the matrix.With the addition of 10 vol.%ZrSi₂ and 10 vol.%h-BN@SiO₂ at the same time,Si₃N₄/TiC/ZrSi₂/h-BN@SiO₂ ceramic tool materials have better comprehensive properties,with bending strength of 702 MPa,fracture toughness of 8.73 MPa·m^1/2 and Vickers hardness of 15.01 GPa.Cracks were introduced into the surface of ceramic tool materials by Vickers hardness tester,and the optimal repair parameters and repair mechanism of cracks in Si₃N₄/TiC-based ceramic materials with ZrSi₂ as repair agent were studied.The bending strength of Si₃N₄/TiC/ZrSi₂ crack specimen with 10 vol.%ZrSi₂ added can be restored to 92.02%of that of smooth specimen after heat treatment at 800?for 60min in air atmosphere,and the crack with the surface length of 300-350?m is completely repaired,and an oxide layer mainly composed of ZrO₂ and SiO₂ is formed.That h-BN@SiO₂ can cooperate with ZrSi₂ to repair cracks.Under the same heat treatment conditions,the flexural strength of the cracked Si₃N₄/TiC/ZrSi₂/h-BN@SiO₂ceramic samples with 10 vol.%h-BN@SiO₂ is basically restored to the strength of smooth samples,and the surface of the samples after heat treatment is covered with a layer of oxide.EDS and XRD show that the composition of oxide layer on the surface of the samples is ZrO₂,SiO₂ and B₂O₃.The cross section of crack specimen shows that the crack interior is repaired,and the thickness of oxide layer is about 4?m.The crack repair mechanism of ceramic tool materials is analyzed as follows:First,in the air heat treatment process,the repair agent ZrSi₂ reacts with oxygen to produce ZrO₂ and SiO₂;Second,h-BN@SiO₂ at the crack fracture site is oxidized to B₂O₃;These three oxides form new chemical bonds with the inner wall of cracks to repair cracks and restore the strength of cracked samples.The friction and wear properties of Si₃N₄/TiC/ZrSi₂/h-BN@SiO₂ ceramic tool materials with different reciprocating frequency,load and h-BN@SiO₂ content were studied.The results show that the Si₃N₄/TiC/ZrSi₂/h-BN@SiO₂ ceramic tool material with 10 vol.%h-BN@SiO₂ has lower friction coefficient and wear rate of 0.45 and0.43×10^-6 mm/Nm,respectively,when the reciprocating frequency is 5 Hz and the applied load is 10 N.SEM and EDS of wear surface show that h-BN in h-BN@SiO₂particles can play a good role in reducing friction.Meanwhile,the existence of SiO₂shell makes up for the decrease of properties of ceramic tool materials caused by direct addition of h-BN,and improves the wear resistance of ceramic tool materials.The cutting performance of Si₃N₄/TiC/ZrSi₂/h-BN@SiO₂ ceramic tools with 10vol.%ZrSi₂ and h-BN@SiO₂ was studied.The main cutting force and cutting temperature of the ceramic tool with 10 vol.%ZrSi₂ and h-BN@SiO₂ are 19.04%and16.42%lower than those of Si₃N₄/TiC ceramic tool,and the surface roughness of machined workpiece is 1.5.The main wear forms of the rake face of the ceramic tool with 10 vol.%ZrSi₂ and h-BN@SiO₂ are crescent wear and bond wear,while the rake face is abrasive wear and bond wear.The anti-friction mechanism is that h-BN and oxidation products SiO₂ and B₂O₃ make the tool self-lubricating and reduce the friction coefficient between the tool and the workpiece.The wear-resistant mechanism is that ZrO₂,SiO₂ and B₂O₃,the oxidation products of ZrSi₂ and h-BN on the tool surface,can repair cracks in time and improve the wear resistance of the tool.Inaddition,h-BN and oxidation products SiO₂ and B₂O₃ make the tool self-lubricating and reduce the friction coefficient between the tool and the workpiece.