Effects Of Si Addition On High Temperature Oxidation And Mechanical Properties Of TC4 Alloy Nickel-based Coatings

The excellent performance of titanium alloy makes it widely used in many engineering fields.Among titanium alloys,TC4 alloy belongs to a widely used alloy.However,it is easily oxidized and fails at high temperatures above 650°C.In view of poor oxidation resistance of TC4 alloy,the composites coatings prepared by laser cladding were used to solve the above shortcoming.A mixture composed of Ni Cr BSi and Si ranging from 5 wt.%to 30 wt.%was employed as the cladding material.The investigation into the evolution in microstructure,mechanical properties and oxidation resistance with the Si content was carried out in detail.Especially,oxidation mechanism was highlighted.The results indicated that phase constituents strongly depended on the Si addition content.The coating with a Si content of 5 wt.%was mainly composed of Ti₂Ni,Ti Ni,Ti B₂,Ti B and Ti C.A new phase of Ti₅Si₃ was synthesized in the coatings with a Si content more than 10 wt.%,and its content presented an upward tendency with the Si content coupled with the increase in Ti B₂,Ti B and Ti Ni.The microstructural change caused the slight increase in microhardness of the coatings,accompanied with which their fracture toughness was correspondingly reduced（an about 3.82%increase in former and an about23.90%decrease in latter with the Si content increased from 5 wt.%to 30 wt.%）.Oxidation resistance of the coatings was evaluated by the oxidation tests performed for 200 h at 800°C.The increase in Si content significantly improved oxidation resistance of the coatings due to the reduction in oxidation weight gain.Two stages can be clearly identified in the whole oxidation process,corresponding to initial stage and slow stage,which were expressed by two general formulas（G=K ₁T,G²=K ₂T）with respect to the relationship between oxidation weight gain（G）and oxidation duration（T）.The coefficient of oxidation weight gain（K₁）in the initial stage was increased along increasing the Si content,however the duration presented the contrary change.The coefficient（K₂）in the slow stage was gradually reduced with increasing the Si addition content.The oxidation weight gain of the coatings was reduced about36.98%when the Si content was increased from 5 wt.%to 30 wt.%.A compacter oxidation layer could be formed on the coating’s surface with a higher Si content due to the higher PBR（Pilling-Bedworth ratio）value for Ti₅Si₃ subject to oxidation than those of the other phases,which played the essential role in the improvement in the coatings’oxidation resistance.For the friction and wear resistance of the coating,when the Si content is less than 20 wt.%,the wear volume of the coating is small.In particular,the wear volume of the coating II is the smallest and the wear volume of the coating V is the largest.Based on the friction and wear curve,the friction coefficient of the coating gradually shows a stable trend when the friction and wear time reaches 60 min.The friction coefficient of coating II is the smallest and that of coating V is the largest.When the Si content is less than 20 wt.%,the friction coefficient of the coating is relatively low.After comparing the trends of the two,it is concluded that the friction coefficient of the coating is positively related to the wear volume of the coating.When the Si content is less than 20 wt.%,the coating has good friction and wear resistance.