Al-Sn plain bearings for automotive applications traditionally comprise a multilayer structure. Conventionally, bearing manufacturing involves casting the Al-Sn alloy and roll¬bonding to a steel backing strip. Recently, high velocity oxy- fuel thermal spraying has been employed as a novel alternative manufacturing route. The present project extends previous work on ternary Al-Sn- Cu alloys to quaternary systems, which contain specific additions for potentially enhanced properties. Two alloys were studied in detail, namely Al-20wt.%Sn-lwt.%Cu-2wt.%Ni and Al-20wt.%Sn-lwt.%Cu-7wt.%Si. This paper will describe the microstructural evolution of these alloys following HVOF spraying onto steel substrates and subsequent heat treatment. Microstructures of powders and coatings were investigated by scanning electron microscopy and phases identified by X-ray diffraction. Coating microhardnesses were determined in both as-sprayed and heat treated conditions and differences related to the microstructures which developed. Finally, the wear behaviour of the sprayed and heat treated coatings in hot engine oil was measured using an industry standard test and compared with that of conventionally manufactured Al-Sn bearings.

A high velocity oxy-fuel spraying system using liquid fuel (HVOLF) has produced Stellite 6 (CoCrWC) and NiCoCrAlY coatings from gas atomised powders of both alloys. Comparative coatings were prepared by weld 0verlay (Stellite 6) and vacuum plasma spraying (VPS) of NiCoCrAlY. The original powders and coatings were characterised by Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD). The presence or absence of cored dendritic microstructures in etched surfaces was used to interpret the degree of melting which took place during spraying, i.e. >80% in VPS and <20% in HVOLF. Low levels of oxide were formed in HVOLF coatings due to the reduced level of thermal transfer during the short heating cycle involved in spraying. For the same reason less melting occurred which permitted sufficient plastic deformation of the solid fraction to occur, reducing voidage between particles and then allowing the liquid fraction to weld them together. This pattern of short cycle heating with both NiCoCrAlY and Stellite 6 particles influenced second phase formation. In NiCoCrAlY coatings the β (Co Ni Al) phase remained as relatively coarse precipitates in the as - sprayed condition whilst in Stellite 6 little carbide eutectic formed. HVOLF spraying can produce coatings with different hardness values when compared with VPS coatings and weld overlays. This may widen the range of coating applications as well as reduce costs.

A number of studies have examined the influence of process parameters on the microstructure and corrosion resistance of metallic coatings produced by gas-fuel HVOF spraying, but much less is known about the coatings produced by liquid-fuel HVOF processes. The aim of this study is to investigate the corrosion behavior and microstructure of nickel and cobalt alloys deposited by both methods and show how it compares with that of various superalloys. Paper includes a German-language abstract.

Zinc and aluminium coatings have been used widely to protect steel structures from corrosion in aggressive and hostile conditions. The more recent development of zinc 15wt% aluminium alloy in a wire form has demonstrated that arc-spray coatings can be produced with a resistance to red rust which is superior to that of the single metals. Competitive 'pseudo' alloy or composite coatings produced by co-spraying wires of zinc and aluminium have been shown to achieve resistance to salt spray conditions similar to this conventional alloy. Work described in this paper confirms these findings and goes on to demonstrate an additional advantage of co-spraying an aluminium -5wt% magnesium instead of aluminium with the zinc. The importance of providing a fine dispersion of the two phases in the 'pseudo' alloy is highlighted and an alternative method of providing a similar dispersion by using a 'cored' wire (e.g. Al-5wt%Mg wire in a zinc sheath) approach has been demonstrated. The importance of 'self sealing' in these coatings after the initial loss of zinc is discussed which is related to the coating microstructure and their electrochemical behaviour in chloride solutions.

Dilute aluminium alloys with additions of tin and indium when deposited by thermal spraying no longer behave as barrier coatings but demonstrate sacrificial corrosion properties when they exist on corrodible substrates. The degree to which the sacrificial attack occurs depends upon the spraying conditions and the tin or indium contents of the coating. The form in which the tin and/or indium exists in these coatings has not been specified but both elements are known to be sparingly soluble in aluminium. A series of experiments have been carried out using Al-12wt%Sn alloy powder as a feedstock for high velocity oxy-fuel (HVOF) spraying on to a steel substrate. The as-sprayed coatings were highly reactive in distilled water and dissolved in a few minutes. Heat-treatment of the coatings at 450°C for increasing amounts of time up to 20 hours reduced the reactivity to water but did not influence the corrosion rate in 0.1M NaCl solution. SEM/TEM observations on the coating provided evidence of the coarsening of tin particles from 15nm (as sprayed) to 0.5-2µm (as heat-treated). A second alloy with a copper addition i.e. Al-12wt%Sn-1wt%Cu was also sprayed to form coatings. The copper addition prevented reaction in water but did not influence the high corrosion rate of the as-sprayed coating in 0.1M NaCl. Heat treatment at 450°C reduced the corrosion rate and allowed passive films to form over limited ranges of electrode potential. The size and distribution of the tin phase was different in the copper containing coatings and this influenced the corrosion rate.

The corrosion characteristics of two bespoke Ni-Cr-Mo-B alloy powders sprayed by the high velocity oxy-fuel (HVOF) process have been studied using potentiodynamic and potentiostatic corrosion analysis in 0.5M H2SO4. The deposits have also been microstructurally characterised using X-ray diffraction (XRD), scanning electron microscopy (utilising both secondary electron (SE) and backscattered electron (BE) modes), and transmission electron microscopy (TEM). Results from the microstructural examination of the two alloys have revealed a predominantly amorphous/nanocrystalline (fcc) matrix containing submicron boride precipitates as well as regions of martensitically transformed laths. Apparent recrystallisation of the amorphous matrix has also been observed in the form of cellular crystals with an fcc structure. The oxide stringers observed at splat boundaries were found to be columnar grained α-Cr 2 O 3 , though regions of the spinel oxide NiCr 2 O 4 with a globular morphology were also observed. The coatings of the two alloys exhibited comparable resistance to corrosion in 0.5M H 2 SO 4 , as revealed by potentiodynamic tests. They both had rest potentials approximately equal to -300mV(SCE) and passive region current densities of around 1mAcm-2. Microstructural examination of samples tested potentiostatically revealed the prevalence of degradation at splat boundaries, especially those where significant oxidation of the deposit had occurred.

Thermal spraying has been used to protect many steel structures from aqueous corrosion using Zinc and Aluminium, and to some extent their alloy coatings to provide galvanic protection. The lifetimes of the coatings can approach 50 years even when exposed in severe marine environments. Zinc coatings work by continuously sacrificing themselves and slowly dissipating over time. Aluminium coatings passivate more readily and form a barrier layer, the passivity makes them less able to protect damaged areas and to self heal. A new ternary coating system involving Aluminium, Zinc and Magnesium has been shown to be capable of providing both a passive barrier layer as well as being able to give galvanically active protection. Salt spray tests have shown that the resistance to red rust of these new coatings increases by 300% over similar thicknesses of the separate metal coatings. Processing by arcspray is straightforward and both adhesion and deposition efficiency are better than where Zinc is sprayed alone.

Two boron-rich Fe/Cr based gas atomised powders (Armacor M and Armacor C) have been thermally sprayed using the HVOF process and the resultant deposits subsequently characterised, using X-ray diffraction, scanning electron microscopy (SEM), plan view transmission electron microscopy (TEM), and microhardness measurements. The wear and corrosion characteristics of the two alloy coatings have also been investigated by three body abrasive wear (utilising cross-sectional TEM to examine the worn surfaces) and potentiodynamic corrosion testing respectively. Results from microstructural analysis of the as-sprayed deposits revealed the presence of small chromium-iron boride precipitates within a predominantly amorphous matrix in the Fe-based Armacor M coating. The Fe-Cr-based Armacor C coating, however, consisted mainly of regions of nano- and microcrystalline material interspersed with chromium boride precipitation. Iron-chromium oxides have been observed within both of the alloy coatings studied. Both of these alloys exhibit good abrasive wear resistance when compared with other metallic based HVOF sprayed coatings. Both Armacor M and Armacor C also exhibit extensive passivation on exposure to an acidic solution. The wear and corrosion test results are related to the microstructural observations.