Microstructure And Wear Properties Of Al₂O₃-13wt%TiO₂ Composite Coatings

The effects of spraying parameters such as current,H₂ flow rate and spraying distance on the structure and properties of micro-structure Al₂O₃-13%TiO₂ coatings were studied.The micro-morphology,phase structure and composition of the coatings were analyzed by means of SEM,XRD and EDS.The formation process and mechanism of micro-Al₂O₃-13%TiO₂coatings were analyzed and discussed.The microhardness,porosity and deposition thickness of the coatings under different spraying parameters were measured,and the dry friction and wear experiments were carried out.The effects of different process parameters on the structure,mechanical properties and wear resistance of the coatings were studied.The relationship between the wear failure behavior of plasma sprayed coatings and their microstructural characteristics was discussed.The wear resistance of the coatings was evaluated according to the weight loss of the coatings,and the wear mechanism of the coatings was analyzed.The results are as follows:1)The phase of the coating is?-Al₂O₃,?-Al₂O₃,Al₂TiO₅ and a small amount of Rutile-TiO₂.?-Al₂O₃ layer is wrapped in?-Al₂O₃,and Al₂TiO₅ is mixed in gamma-Al2O3layer.The layer structure of the coating is?-Al₂O₃ layer-?-Al₂O₃ layer-Al₂TiO₅ layer-?-Al₂O₃layer-?-Al₂O₃ layer.Formation process of coating:the powder of?-Al₂O₃ in coating feed melts to a full or semi-melted state in high temperature plasma arc,the melting zone of powder sprayed to the substrate is impacted and spread out,part of the phase of?-Al₂O₃ is transformed into?-Al₂O₃ phase in melting recrystallization,and voids are formed in rapid stacking and lapping with the micro-concave of the substrate coating,and the fully melted TiO₂ powder is formed during stacking process.Al₂TiO₅ phase was formed by solid solution.Most of the voids exist between the layers of?-Al₂O₃ and Al₂TiO₅.2)With the increase of plasma arc power,the metallurgical bonding ratio of the coating increases and the coating tends to be compact.At the same time,the transformation of?-Al₂O₃ crystal to?-Al₂O₃ crystal increases,the diffraction peak of Al₂TiO₅ increases,the crystallinity of the crystal increases,and the number of pore in the coating decreases.When the plasma arc power is low,the particle melting is insufficient and the deposition efficiency is poor.Excessive high plasma arc power will increase the thermal stress and phase transformation on the coating performance.When the plasma arc power is between 2800 W and 3000w,the melting state of the coating is the best.There are many phases of?-Al₂O₃ in the coating,and the micro-hardness and wear resistance of the coating are higher.3)The spraying distance has a great influence on the properties of the coating.The increase of the spraying distance will lead to the increase of the transformation from?-Al₂O₃crystal to?-Al₂O₃ crystal.At the same time,the microhardness of coating section increases slowly at first and then decreases rapidly.When the mass percentage of?-Al₂O₃ phase is small,the microhardness decreases obviously;the deposition efficiency of the coating decreases gradually;the porosity decreases first and then increases;the void ratio of the coating increases with the rapid stacking of insufficient melting and the insufficient impact velocity of sufficient melting.The wear resistance of the coating is 130 mm,120 mm,110mm,140 mm and 150 mm respectively.The melting of 110 mm coating is poor and easy to peel and scratch to produce furrows.There are more?-Al₂O₃ phases in 150 mm coating,and the wear morphology shows brittle fracture.130 mm coating has good wear resistance,and the main failure mode is micro ploughing.When the spraying distance is small,the particle retention time is short,the melting rate is insufficient,the stacking speed is large,the pore is easy to form,the stress concentration is easy to occur in wear and the wear resistance is reduced;when the spraying distance is large,the velocity of melt droplet impacting the matrix is reduced,the coating stacking is not well spread,the porosity is high,and the melting degree increases,which makes the phase of?-Al₂O₃ increase,the brittleness of the coating is remarkable and the wear resistance When the hard phase of?-Al₂O₃ melts well,it is not easy to be peeled off,which plays a load-bearing role,reduces the damage of the wear pair and debris friction to the coating,and enhances the wear resistance.