SPS Preparation Process And Properties Of Surface Reinforced Metal Matrix Composites

Surface modification technology is to modify the surface of materials,using this technology can give or strengthen some properties of the surface of materials.Thermal diffusion method is one of the common surface modification techniques,which can obtain diffusion layer on the substrate with high bonding strength and easy to operate.Therefore,adopt thermal diffusion method to improve the surface hardness to broaden application range,has a significant research value.In this paper,the surface modification of CP-Ti（TA1）,45 and Q235 steels by Spark Plasma Sintering（SPS）was studied.Firstly,graphene（Gr）,Nano-diamonds（NDs）,Ti B₂ and Zr O₂/Ti thermal diffusion samples were prepared by SPS at 1273 K for 30 min.The phase composition,microstructure and mechanical properties of different samples were discussed in detail.During SPS thermal diffusion,the addition amount of Gr is only 0.05 g,resulting the maximum cross section hardness of Gr/Ti obtained is only 315 HV_0.2.NDs produces discharge effect which forming irregular islands on the surface of Ti.The Zr O₂ thin layer of 20～30μm is formed on the surface of Zr O₂/Ti,and the hardness is up to 768 HV_0.2.There are two boride layers formed on the surface of Ti B₂/Ti,Ti B₂ layer and Ti B whisker（Ti B_w）layer.The Ti B_w layer is composed of Ti B_w and the Ti matrix included in it.The hardness of Ti B₂ layer is 1120 HV_0.2,and the hardness of Ti B_wlayer is 656 HV_0.2.The core of the four thermal diffused samples still maintains good plasticity and toughness.Based on the above discussion,the boride layer of Ti B₂/Ti sample has obvious structure and the best performance.In the following chapter,the effects of SPS temperature（1123/1173/1223/1273 K）and holding time（10/20/30 min）on the morphology,thickness,growth kinetics and hardness of boride layer were studied systematically.The results show that the formation process of boride layer is as follows:（1）Ti B₂ powders in contact with the Ti substrate are reacted with Ti to form Ti B whiskers.（2）B atoms in other parts of Ti B₂ are activated by high temperature and plasma to form active boron atoms.（3）The active B atoms diffuse through Ti B₂ and Ti B,part of which react with Ti to thicken Ti B₂ layer,and part of which diffuse to the bottom of Ti B whisker and then travelled along Ti B whisker to the top.Finally,the B atoms are reacted with Ti to form new Ti B achieving a continuous growth of Ti B whisker.The maximum thickness of Ti B₂ layer and Ti B_wlayer is 2.24±0.49μm and 15.63±6.97μm,respectively.The analysis of the growth kinetics of the boride layers show that the diffusion activation energies of B atoms in the Ti B₂ layer and the Ti B_w layer are 386.49k Jmol^-1 and 343.57 k Jmol^-1,respectively.In addition,the boride layer still grows well in poor vacuum atmosphere with a vacuum level of 6 Pa（oxygen partial pressure about 0.6 Pa）,indicating that the high-temperature plasma generated by the pulsed DC in SPS can effectively prevent the oxidation of Ti surface to form Ti O₂.In hardness test,the surface hardness of Ti B₂/Ti sample is 3.4～6 times higher than that of CP-Ti,and the effect is obvious.Finally,the effect of SPS on carburization of carbon steel was explored,and the effect of different carbon sources on the carburizing layer was discussed.After SPS carburization,800μm carburizing layer is obtained in Q235 steel,the top of which is composed of cementite and pearlite.When NDs is used as carbon source,800μm carburizing layer is obtained in Q235 steel and 45 steel.When graphite is used as carbon source,only 500μm carburizing layer can be obtained in graphite/45 steel sample.In the hardness test,the hardness of graphite/Q235 steel and NDs/Q235 steel prepared by SPS carburization is 334 HV_0.2,which is 2.8 times that of the matrix.The surface hardness of graphite/45 steel and NDs/45 steel are 357 HV_0.2 and 402 HV_0.2,respectively,with obvious strengthening effect.After quenching and tempering,cementite breaks or combines with ferrite to become pearlite,so the hardness value decreases.The surface hardness of graphite/Q235 steel,NDs/Q235 steel,graphite/45 steel and NDs/45 steel sample are 260 HV_0.2,278 HV_0.2,274 HV_0.2and 327 HV_0.2,respectively.Considering the change of carburizing layer thickness and hardness,NDs as carbon source can obtain higher hardness,and has better strengthening effect.