High temperature tribological performance of nickel-based composite coatings by incorporating multiple oxides (TiO₂–ZnO–MoO₃)

Highlights

• Ni-based coatings with TiO₂–ZnO–MoO₃ multiple oxides were prepared.

• Composite coatings demonstrated the improved lubricating capability at 800 °C.

• Molybdates and titanates were formed during the spraying process and sliding test.

• The tribo-layers after wear test at 800 °C were analyzed.

Abstract

The potential of lower friction and desired wear for nickel-based composite coating system at high temperatures was explored, where multiple oxides were incorporated into nickel matrix during the atmospheric plasma spraying process. The phase composition and microstructure of wear track were characterized by XRD, Raman, SEM/EDS, and FIB/STEM. The further addition of MoO₃ allows for an effective reduction of friction coefficient at 800 °C. The surficial and interfacial analysis well confirmed that a protective tribo-layer presenting on the contacting surface, in which binary oxides (A-TiO₂, R–TiO₂, ZnO, NiO), molybdates and titanates (ZnMoO₄, NiMoO₄, and Zn₂TiO₄) were identifiable. The association of friction and wear properties with interface chemistry was discussed for the nickel-based coatings with the addition of multiple oxides (TiO₂–ZnO–MoO₃).

Introduction

Ternary and binary oxide phases are always added in bulk materials directly by powder metallurgy or deposited as coatings, and they have been widely studied as high temperature lubricants [1]. It has been revealed that the synergistic effect of binary oxides and newly in-situ formed ternary oxides plays the dominant role in improving tribological properties [[2], [3], [4], [5], [6]]. For example, ternary oxides like silver molybdates (Ag₂MoO₄, Ag₂Mo₂O₇, and Ag₂Mo₄O₁₃) have inherently lamellar structure and thereby, are very lubricious [[7], [8], [9]]. As highlighted that, the Mo–Ag–O system renders excellent lubricating performance from 350 °C to 600 °C, which has been reported to be associated with high temperature reaction, tribo-chemical processes, sublimation or melting [[10], [11], [12]]. Considering the melting temperature, Ag₂Mo₂O₇ is stable up to 400 °C and will segregate into MoO₃ and Ag from 400 °C to 500 °C [9]; The eutectic mixture of Ag₂Mo₄O₁₃ and Ag₂Mo₂O₇ has the melting temperature of about 508 °C, while the melting temperature of another eutectic mixture of Ag₂Mo₂O₇ and Ag₂MoO₄ is 496 °C [13].

The tribological properties of binary oxides systems have also been studied, e.g. MoO₃–PbO, MoO₃–CuO, MoO₃–NiO, and MoO₃–ZnO system [[14], [15], [16], [17], [18], [19]]. In comparison, CuMoO₄, Cu₃Mo₂O₉ and ZnMoO₄ compounds have a relatively higher melting point, about 820 °C, 855 °C, and 1003 °C, respectively [[20], [21], [22]]. Ouyang et al. fabricated self-lubricating NiCr–BaMoO₄ composites by powder metallurgy method with friction coefficients less than 0.30 at high temperatures [2]. Nickel-based coatings containing nanostructured TiO₂/Bi₂O₃ exhibited excellent tribological behavior with the friction coefficient of 0.07 at 800 °C, which was attributed to the synergistic effect of ternary oxides (Bi₄Ti₃O₁₂ and NiTiO₃) and binary oxides (TiO₂ and NiO) on the worn surfaces [3]. Moreover, T.W. Scharf et al. [4,23] found that ZnTiO₃ with the (104) stacking faults was able to be sheared parallelly to the sliding direction and thus atomic layer deposition ZnTiO₃ coating displayed better tribological performance. Very recently, Ni–5wt.%Al/TiO₂–ZnO coatings were prepared by atmospheric plasma spraying (APS), the improvement in tribological performance of nickel-based matrix attained as the temperature exceeds 600 °C [24].

In this study, we planned to make a continuity to study the tribological potentials of composite system by virtue of unique structural characteristic and broad melting temperature of molybdate compounds. Thus the Ni–5wt.%Al/TiO₂–ZnO composite coatings with additional addition of MoO₃ were developed by the APS process. Different from the previous work [24], TiO₂ and ZnO (1:1 in wt.%) mixed powders were pre-agglomerated by the solution of 5 wt.% polyvinyl alcohol (PVA) in this work. The effect of the MoO₃ addition on the tribological behaviors of Ni–20 wt.% TiO₂/ZnO composite coatings was assessed by the high-temperature ball-on-disc testers. Surface and interface characterizations were implemented to analyze the morphology and constituent of wear tracks and relevant debris, to reveal the underlying mechanism on the view of oxidation and tribo-chemistry between the sliding contact surface.