Tribology International
Influence of doping graphite on microstructure and tribological properties of plasma sprayed 3Al2O3–2SiO2 coating
Abstract:With the combination of self-lubricating ability and mechanical property, high-performance ceramic-graphite composite coatings are potential candidates for machine components. Mullite/nickel cladded graphite (Ni–graphite) composite coatings with different content of graphite were fabricated by air plasma spraying technique. Results show that doping a suitable amount of Ni–graphite could greatly reduce the defects of mullite coating without significant decrease of their hardness. With the formation of re-graphitization lubricating film on friction surface, the friction coefficient of composite coating with 20 wt% Ni–graphite was observed to be 0.28, which was approximately 52% lower than that of pure mullite coating, and its wear rates was 4.15 times lower than that of pure mullite coating.
1.Introduction
Plasma spraying is well known for its ability to make functional metallic or ceramic coatings to improve the performance or pro-long the service life of components [1,2]. With the outstanding physical and chemical properties, oxide ceramics are often deposited onto metal substrates to improve their wear, corrosion or thermal resistance properties [3–7]. Actually, because of its high hardness, plasma sprayed ceramic/cermet coatings exhibit better resistance to abrasion than traditional hard chromium electro-plating [8,9]. Notably, benefited from the ultra-high temperature of plasma core (up to 15,000 K), a wide varieties of materials could be melted and deposited on substrate. Compared to hard phase, the softer material (graphite, Polytetrafluoroethylene, etc.) would cause less damage to the counterpart. Therefore, to mitigate the worn of frictional counterpart, some soft materials (e.g. aluminum bronze [10], poly-(p)-oxybenzoyl [11]) with excellence self-lubricating ability can also be deposited with or without hard load bearing phase by plasma spraying technique.
Plasma sprayed mullite (3Al2O3–2SiO2) and mullite-based coatings are being widely developed as thermal and environ-mental barrier coatings (T-EBCS) for application in aerospace industry [12–14]. According to our previous researches, under a given sliding condition, the mullite coating exhibited ultra-low wear rate ( 2.0 10 6 mm3/N m) with a coefficient of friction around 0.4 [15]. However, the working condition of component is variable, and the adaptation of various service conditions is the principal requirement of materials for safe and high efficiency work. Generally, different friction conditions always result in diverse results [16,17]. An et al. [18] investigated the tribological behaviors of plasma sprayed mullite coating under high tem-peratures. They indicated that mullite coatings suffer severe wear, and their wear rate reached to 8.6 10 5 mm3/N m with the coefficient of friction of about 1.12. Thus, as a promising candidate of wear-resistant material, the tribological properties of mullite should be investigated under different friction conditions, for example, different loads.
It is commonly accepted that the increased wear resistance of a coating should not at the cost of increasing wear damage to its frictional counterpart. A useful method to minimize the friction induced damage or devices failure during sliding of machine components is doping solid lubricants into hard coatings [16,19]. In this respect, high performance ceramic-graphite composites are promising candidates for solving the problem, since graphite can act as a lubricant to form a lubricating film on the surfaces of ceramics during sliding process [20–23]. However, since graphite is apt to oxidization under high temperatures with the existence of oxygen, few researches were reported that ceramic-graphite composites coatings were produced by atmospheric plasma spraying (APS) technology.
Addressing these issues, in the present work, APS technique was adopted to incorporate different contents of graphite into mullite matrix by adjusting spraying parameters and selecting nickel cladded graphite as graphite source. The followings are the main objectives of the present investigation: (i) fabricate different proportion of mullite/Ni–graphite coatings; (ii)analyze the effect of doping Ni–graphite on the microstructure of mullite coating; (iii) investigate the tribological properties of pure mullite coating under different loads; (iv) study the wear mechanisms of pure mullite and mullite/Ni–graphite composite coatings.
2 .Experimental
2.1. Preparation of mullite/mullite based coatings
The feedstock powders were prepared by mechanically blend-ing different mass fractions of mullite (Metco 6150, Sulter, Swit-zerland) and nickel cladded graphite (KF21R, Beijing Research Institute of Mining amp; Metallurgy, China), with the composition of 20 wt% Graphite and 80 wt% Nickel. The detailed compositions of hybrid composition coatings were 0, 10, 20 and 30 wt% Ni–gra-phite, designated as coating 1, coating 2, coating 3 and coating 4, respectively.
The coatings were deposited onto Austenitic stainless steel substrates (1Cr18Ni9Ti, 24 mm 24 mm 7.8 mm) with a thick-ness of 350 mu;m by an APS-2000A system (Institute of Aero-nautical Manufacturing Technology, Beijing, China). To precise control the spraying distance and passes, the plasma gun was fixed at a six-axis robot (IRB 2400, ASEA Br
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Tribology International
Influence of doping graphite on microstructure and tribological properties of plasma sprayed 3Al2O3–2SiO2 coating
a b s t r a c t
With the combination of self-lubricating ability and mechanical property, high-performance ceramic-graphite composite coatings are potential candidates for machine components. Mullite/nickel cladded graphite (Ni–graphite) composite coatings with different content of graphite were fabricated by air plasma spraying technique. Results show that doping a suitable amount of Ni–graphite could greatly reduce the defects of mullite coating without significant decrease of their hardness. With the formation of re-graphitization lubricating film on friction surface, the friction coefficient of composite coating with 20 wt% Ni–graphite was observed to be 0.28, which was approximately 52% lower than that of pure mullite coating, and its wear rates was 4.15 times lower than that of pure mullite coating.
1.Introduction
Plasma spraying is well known for its ability to make functional metallic or ceramic coatings to improve the performance or pro-long the service life of components [1,2]. With the outstanding physical and chemical properties, oxide ceramics are often deposited onto metal substrates to improve their wear, corrosion or thermal resistance properties [3–7]. Actually, because of its high hardness, plasma sprayed ceramic/cermet coatings exhibit better resistance to abrasion than traditional hard chromium electro-plating [8,9]. Notably, benefited from the ultra-high temperature of plasma core (up to 15,000 K), a wide varieties of materials could be melted and deposited on substrate. Compared to hard phase, the softer material (graphite, Polytetrafluoroethylene, etc.) would cause less damage to the counterpart. Therefore, to mitigate the worn of frictional counterpart, some soft materials (e.g. aluminum bronze [10], poly-(p)-oxybenzoyl [11]) with excellence self-lubricating ability can also be deposited with or without hard load bearing phase by plasma spraying technique.
Plasma sprayed mullite (3Al2O3–2SiO2) and mullite-based coatings are being widely developed as thermal and environ-mental barrier coatings (T-EBCS) for application in aerospace industry [12–14]. According to our previous researches, under a given sliding condition, the mullite coating exhibited ultra-low wear rate ( 2.0 10 6 mm3/N m) with a coefficient of friction around 0.4 [15]. However, the working condition of component is variable, and the adaptation of various service conditions is the principal requirement of materials for safe and high efficiency work. Generally, different friction conditions always result in diverse results [16,17]. An et al. [18] investigated the tribological behaviors of plasma sprayed mullite coating under high tem-peratures. They indicated that mullite coatings suffer severe wear, and their wear rate reached to 8.6 10 5 mm3/N m with the coefficient of friction of about 1.12. Thus, as a promising candidate of wear-resistant material, the tribological properties of mullite should be investigated under different friction conditions, for example, different loads.
It is commonly accepted that the increased wear resistance of a coating should not at the cost of increasing wear damage to its frictional counterpart. A useful method to minimize the friction induced damage or devices failure during sliding of machine components is doping solid lubricants into hard coatings [16,19]. In this respect, high performance ceramic-graphite composites are promising candidates for solving the problem, since graphite can act as a lubricant to form a lubricating film on the surfaces of ceramics during sliding process [20–23]. However, since graphite is apt to oxidization under high temperatures with the existence of oxygen, few researches were reported that ceramic-graphite composites coatings were produced by atmospheric plasma spraying (APS) technology.
Addressing these issues, in the present work, APS technique was adopted to incorporate different contents of graphite into mullite matrix by adjusting spraying parameters and selecting nickel cladded graphite as graphite source. The followings are the main objectives of the present investigation: (i) fabricate different proportion of mullite/Ni–graphite coatings; (ii)analyze the effect of doping Ni–graphite on the microstructure of mullite coating; (iii) investigate the tribological properties of pure mullite coating under different loads; (iv) study the wear mechanisms of pure mullite and mullite/Ni–graphite composite coatings.
2 .Experimental
2.1. Preparation of mullite/mullite based coatings
The feedstock powders were prepared by mechanically blend-ing different mass fractions of mullite (Metco 6150, Sulter, Swit-zerland) and nickel cladded graphite (KF21R, Beijing Research Institute of Mining amp; Metallurgy, China), with the composition of 20 wt% Graphite and 80 wt% Nickel. The detailed compositions of hybrid composition
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