Transitions between two-body and three-body abrasive wear: influence of test conditions in the microscale abrasive wear test

Since the inception of electroless coating by Brenner & Riddell in 1946, it has been the subject of research interest and, in the past two decades, emphasis has shifted to the studies of its properties and applications. The co-deposition of paniculate matter or substance within the growing film has led to a new generation of electroless composite coatings, many of which possess excellent wear and corrosion resistance. This valuable process can coat not only electrically conductive materials including graphite but also fabrics, insulators like plastics, rubber etc. The low coating rates with these can provide better reflectivity of plated surfaces and many more applications. Coatings can be tailored for desired properties by selecting the composition of the coating alloy/composite/metallic to suit specific requirements. The market for these coatings is expanding fast as the potential applications are on the rise. In the present article, an attempt has been made to review different electroless alloy/composite coatings with respect to bath types and their composition, properties and applications. Different characterisation studies have been conducted on various electroless nickel-based coatings with emphasis on wear and corrosion properties.

Electroless alloy/composite coatings: A review

The present work aims to study the hardness and wear resistance of electroless Ni–B coatings. An alkaline bath having nickel chloride as the source of nickel and borohydride as the reducing agent was used to prepare the electroless Ni–B coatings. The structure, microhardness and wear resistance of electroless Ni–B coatings, both in as-plated and heat-treated conditions, were evaluated using X-ray diffraction (XRD), Leitz microhardness tester and a pin-on-disc wear test apparatus. XRD patterns reveal that electroless Ni–B coatings are amorphous in as-plated condition and undergo phase transformation to crystalline nickel and nickel borides upon heat-treatment. The microhardness of the electroless Ni–B coatings increases with increase in heat-treatment temperature and exhibit two maxima in the hardness vs. heat-treatment temperature curve. The specific wear rate increases with increase in applied load from 20 to 40 N and at all applied loads, the specific wear rate and coefficient of friction are less for heat-treated electroless Ni–B deposits compared to that obtained for as-plated ones. The wear process of electroless Ni–B coatings is governed by an adhesive wear mechanism.

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Electroless Ni-B coatings: preparation and evaluation of hardness and wear resistance

Wear-mode mapping for the micro-scale abrasion test

Sliding and abrasive wear behaviour of boride coatings

An electroless NiP coating has been deposited onto an AISI 1020 plain carbon steel by using an acid bath based on NiCl 2 , as source of nickel cations. The tribological behavior under dry non-lubricated conditions has been evaluated by employing the pin-on-disc test, where the pin was made of AISI 52100 steel. The effects of the heat treatment carried out at 400 and 260°C for 1 and 25 h, respectively on the hardness, friction and wear behavior of the coatings were investigated and compared against the properties of the unplated and as-deposited samples. Both optical and electron microscopic techniques were used to study the wear surface. It was shown that the wear resistance was greatly increased with both heat treatments, the treatment performed at 400°C being a little more effective, although no influence was observed on the frictional coefficient.

Wear performance and mechanism of electroless NiP coating

Effect of thermal history on the microhardness of electroless Ni–P

Electroless Ni-P coatings on plain carbon steel were prepared by using an acid bath based on NiCl2 as source of nickel, cations. Heat treatments were performed at 260 ° and 400 °C for 25 h and 1 h, respectively. The abrasive resistances of the coatings which suffered different heat treatments were determined by using the modified ball cratering method, recently proposed by Staia et al. The abrasive media employed was a mixture of 100 ml ethylene glycol and 1 g of 3 μm diamond paste. A RB38.1/G100P3 ball-bearing was used as tribological pair. The individual wear constants for the substrate and coatings were determined from the recorded plots of the crater depth vs. sliding distances with the aid of theory of the imposed shape wear scar reported in the literature by Kassman et al. Electron microscopy was used to study the morphology of the worn surfaces. As a result, it was found that the heat treatment decreases the abrasive wear resistance of the coatings. A comparison between the methods proposed by Rutherford and Hutchings and ours for assessing the wear constants is carried out, particularly in relation to the relevance of including in the computation the value of the friction force.

Influence of the heat treatment on the abrasive wear resistance of electroless Ni-P

Fatigue tests under rotating bending conditions have been conducted on samples of AISI 1045 plain carbon steel with and without the application of electroless Ni–P (EN) deposits of relatively high P content and thicknesses ranging between 7 and 37 μm. The coated samples have been post-heat treated (PHT) according to two different schedules: 473 K for 60 min (PHT-1) and 473 K for 60 min followed by a second treatment at 673 K for 60 min (PHT-2). The mean microhardness values determined on the surface of the coated samples after both post-heat treatments (PHT) were found to be about 675 and 1044 KHN 50 , respectively. In relation to the tensile properties of the coated and PHT specimens it has been determined that both the yield stress and ultimate tensile stress (UTS) decrease slightly as the deposit thickness increases, whereas the reduction in area is severely decreased, independently of the post-heat treatment applied. In relation to the fatigue behavior of the coated and PHT samples, it has been observed that the substrate with a deposit 7 μm thick after PHT-1 behaves similarly to the uncoated substrate. However, as the thickness deposit increases the fatigue life is reduced. The decrease in fatigue life is observed to be more marked for those materials with PHT-2 in comparison with those with PHT-1.

Effect of the thickness of an electroless Ni-P deposit on the mechanical properties of an AISI 1045 plain carbon steel

The objective of the present work is to investigate the morphology, structure and local chemistry of TiN produced by CVD on a 316L stainless steel substrate, chemically etched in situ by using gaseous HCl. The samples were obtained by a reduced pressure CVD process at temperatures of 1173 and 1273 K. The microstructural, chemical characterization and texture of the TiN coatings were performed by using scanning transmission electron microscopy (STEM), scanning electron microscopy (SEM), discharge glow spectroscopy (GDEOS) and X-ray diffraction (XRD), respectively. It was determined that the coatings obtained presented a stoichiometric composition and the interface was free of any contaminants. As the concentration of TiCl 4 increased the texture of the coatings presented a stronger component along the 110 planes and a minor component along 111 planes. In general, large grain sizes were obtained due to a reduced pressure in the system, high temperature employed during the deposition process and subsequent annealing. The grain size of the TiN crystallites becomes smaller as the partial pressure of TiCl 4 decreases. The STEM studies have shown the presence of two kinds of grain structures: a region close to the interface with very fine equiaxed grains from which tall steep columnar grains have been nucleated. The structure observed was dense and without pores.

E.S. Puchi

Papers

Wear performance and mechanism of electroless NiP coating

Effect of thermal history on the microhardness of electroless Ni–P

Influence of the heat treatment on the abrasive wear resistance of electroless Ni-P

Effect of the thickness of an electroless Ni-P deposit on the mechanical properties of an AISI 1045 plain carbon steel

Microstructural characterization of chemically vapor deposited TiN coating