| Erosion is one of major reason for material failure. Erosion-resistant coatings prepared on the carbon steel surface by means of surface engineering technology can effectively reduce materials erosion lost. Plasma spraying is a surface engineering technology often used for preparing abrasion-resistant coating, and widely used in many fields, such as aerospace, metallurgical, energy and mechanical industries due to its unique advantages which include wide sprayed materials, fine coating structure, high bonding strength and high controlling precision. Metal matrix composites (MMC`s) were widely used as abrasion-resistant coating because of the combination of high hardness and high toughness. Nickel base self melting alloy has a excellent wear resistance and a good spraying performance, and is widely used to prepare abrasion-resistant coatings. In addition, it is also a suitable matrix material for MMC`s. The reinforcements of MMC`s can be high hardness solid particles and short fibers. Oxide, nitride and carbonitride ceramics were often used as nonmetallic reinforcement materials. The WC has a high hardness (1300～1800 HV) and similar physical and chemiscal properties to metals, and is widely used as coating reinforcement materials. Moreover, Nickel base self melting alloy is also desirable matrix material of coatings for its excellent mechanical properties and good wetting property. However, the application of WC used in MMC`s was limited, due to its higher price. The Al2O3 ceramics is similar to WC in terms of mechanical properties; besides, Al2O3 ceramics is low cost and considered as a reinforcement material with bright prospect of applications. However, it is difficult to obtain high quality composite coatings using Al2O3 and NiBCrBSi mixed power by means of thermal spray due to the significant difference of density between Al2O3 and NiBCrBSi.WCp/NiCrBSi composite coatings were deposited successfully on the surface of Q235 hot rolling plate by means of plasma spraying, using mixed WC+NiCrBSi powders. The microstructures and properties of WCp/NiCrBSi coating were investigated by means of SEM, XRD, LSCM, VIDAS, microhardness tester and erosion wear test machine.The effects of mass frcation of WC reinforcement particles on the microstructure and properties wrer also investigated. The erosion mechanisms were discussed on the basis of the SEM images of coating erosion morphologies The results show that the WCp/NiCrBSi composite coatings consist mainly of WC,γ-Ni, Ni3B, CrB, Cr2B, M7C3, M23C6 and W2C phases. The WC reinforced particles distributed between splats and in matrix. The W2C content increases with increasing WC mass fraction in the powders. The reinforce particle content has considerable influence on the microstructure and erosion resistance of the WCp/NiCrBSi composite coating. The excessive WC results in decreasing the microhardness due to increasing the porosity. The WCp/NiCrBSi coating with 35% WC mass fraction powder has relatively fine microstructure and more excellent erosion resistance. The result shows that the erosive angle has some influence on the erosion resistance of WCp/NiCrBSi coatings. With an increase of impact angles from 15°to 90°the erosion rate of the coating increases, the erosion rate at 15°impact angle being approximately two times lower than that at 90°impact angle. The results show that under the conditions of small-angle erosion the erosion of the WCp/NiCrBSi coating was mainly associated with that the NiCrBSi matrix was micro cut by erosion particles, which causes the loss of WC hard reinforcements because of lack of brace. In the large-angle erosion conditions, the erosion of the composite coating is mainly attributed to that the cracks easily initiated and propagared within the reinforcement/matrix interface, resulting in the coating material lost. Matlab software programming results and test resules show that the coating deposited under the conditions of the spraying current of 380 A, H2 gas flow rate of 1.5 L/min, powder feed rate of 42 g/min, WC mass fraction of 35% hase the best erosion rate.For the purpose of developing an erosion-resistance composite coating with low cost, NiCrBSi coating and Al2O3 coating were deposited on the Q235 hot rolling plate. The microstructures and properties of NiCrBSi and Al2O3 coatings were investigated. The effects of plasma spraying parameters on the microstructure and properties of NiCrBSi and Al2O3 coatings were also investigated. The erosion mechanisms were discussed. On this basis, Al2O3p/NiCrBSi composite coatings were deposited successfully on the surface of Q235 hot rolling plate by three different plasma spraying processes which include double feeding powders (Al2O3 and NiCrBSi), high energy ball milling powder (Al2O3 and NiCrBSi) and mixed power of Al2O3-Ni and NiCrBSi. The influences of reinforcement content on the coating microstructure and the erosion resistance were evaluated, and erosion mechanism was also discussed. The plasma spraying parameters were optimized by means of the quadratic regression orthogonal combination design and the quadratic regression equation of erosion loss rate of plasma sprayed Al2O3p/NiCrBSi composite coatings was established. The optimum spraying parameters corresponding to the minimum erosion lost rate were obtained by using Matlab software. This work provides scientific basis and technology data for practical application of Al2O3p/NiCrBSi composite coatings.The results show that the NiCrBSi coating is mainly composed ofγ-Ni, Ni3B, Cr2B, CrB, M7C3, M23C6 phases. The plasma spraying parameters (such as HFR and I) and erosive angle have the influence on the properties of plasma sprayed NiCrBSi coatings. With increasing HFR and I, the lamellar thickness in the coating decreases, and the crystallinity, average grain size, micro-strain, residual stress, micro-hardness and erosion resistance of the coating increase. Excessive HFR and I result in increasing coating porosity and decreasing the micro-hardness and erosion resistance of the coating. It was found that the erosion resistance of NiCrBSi coating was higher under the condition of small-angle erosion when HFR was 2 L/min and I was 500 A. The result shows that the erosive angle has some influence on the erosion resistance of NiCrBSi coatings. With an increase of impact angles from 15°to 90°the erosion rate of the coating increases, the erosion rate achieves the maximum at 90°impact angle. The results show that under the conditions of small-angle erosion the erosion of the NiCrBSi coating was mainly associated with the microcut, microplough by erosion particles. In the large-angle erosion conditions, the erosion of the coating is mainly attributed to that the cracks initiated and propagares within the interlamellar, resulting in the coating material lost.The results show that the Al2O3 coating is mainly composed ofα-Al2O3,γ-Al2O3,γ-Ni, Ni3B, Cr2B, CrB, M7C3, M23C6 phases. The plasma spraying parameters (such as HFR and I) and erosive angle have the influences on the properties of plasma sprayed NiCrBSi coatings. With increasing HFR and I, the lamellar thickness in the coating decreases, and the crystallinity, average grain size, micro-strain, residual stress, micro-hardness and erosion resistance of the coating increase. Excessive HFR and I result in higher residual stress of coating and decreasing erosion resistance of the coating. It was found that the erosion resistance of NiCrBSi coating was higher under the condition of small-angle erosion when HFR was 5 L/min and I was 600 A. The result shows that the erosive angle has some influence on the erosion resistance of Al2O3 coating. With an increase of impact angles from 15°to 90°the erosion rate of the coating increases, the erosion rate achieves the maximum at 90°impact angle. The results show that under the conditions of small-angle erosion the erosion of the Al2O3 coating was mainly associated with the microcut by erosion particles and accompanied by fatigue. In the large-angle erosion conditions, the erosion of the coating is mainly attributed to fatigue.The results show that the Al2O3p /NiCrBSi composite coating consists mainly ofα-Al2O3,γ-Al2O3,γ-Ni, Ni3B, CrB, Cr2B, M7C3, M23C6 phases. The Al2O3 reinforced particles distributed between splats and in matrix. It can be concluded from the results that the reinforced particle content has considerable influence on the microstructure and erosion resistance of the Al2O3p/NiCrBSi composite coating. Double feeding powder coating (D25), high energy ball milling powder coating (Q22) and Al2O3-Ni coating (AN60) have relatively fine microstructure and doog anti-erosion performance when Al2O3 contents were 25%, 22%, 60%, respectively. The results show that the structure of powder has significant effect on the microstructure and properties of the Al2O3p/NiCrBSi coatings. The Al2O3p/NiCrBSi deposited by Nickel-coated alumina mixed powder has lower porosity, and higher hardness and erosion resistance. The result shows that the erosive angle has some influence on the erosion resistance of Al2O3p/NiCrBSi coating. With an increase of impact angles from 15°to 90°the erosion rate of the coating increases, the erosion rate at 15°impact angle being approximately three times lower than that at 90°impact angle.The results show that under the conditions of small-angle erosion the erosion of the composite coating was mainly associated with that the NiCrBSi matrix was micro cut by erosion particles, which causes the loss of hard reinforcements because of lack of brace. In the large-angle erosion conditions, the erosion of the composite coating is mainly attributed to that the cracks easily initiated and propagared within the reinforcement/matrix interface, resulting in the coating material lost. Matlab software programming results show that the coating fabricated under the conditions of the spraying current of 350 A, H2 gas flow rate of 2.6 L/min, powder feed rate of 27 g/min and Al2O3-Ni mass fraction of 60% has the lowest erosion rate. The results show that Al2O3p/NiCrBSi composite coating has similar erosion resistance to the WCp/NiCrBSi composite coating, and it cheaper compared with WCp/NiCrBSi coating.
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