材料科学与工程外文翻译--硼化物涂层的滑动和磨粒磨损行为(编辑修改稿)

材料科学与工程外文翻译--硼化物涂层的滑动和磨粒磨损行为(编辑修改稿)内容摘要：

lot of work has been carried out on the tribological behaviour and in particular on the wear resistance of borided steels of very different position, investigated under very different testing conditions. The main interest has been focused on two peculiar characteristics of the boride coatings:(i) high hardness, that is expected to give a high wear resistance, and (ii) columnar morphology, that is required for a good adhesion between coating and substrate. Less attention has been devoted to clarify the mechanisms of wear damage, and in particular to the role of crystallographic orientation of iron borides within the coating. It is well known, in fact, that thermochemical boriding can give rise to iron borides Fe2B (tetragonal) and FeB (orthorhombic), both generally displaying a strong (0 0 2) preferred orientation [2]. The texture strength is significantly influenced by the position of the base alloy, as a consequence of diffusion phenomena involving alloying elements that enter substitutionally the coating from the substrate and modify the properties of both coating and substrate [6]. On the other hand, it has been pointed out that the outermost, few micrometers thick region of the boride coatings is crystallographically disordered and, consequently, it should be removed from the borided ponent by means of a finishing procedure [7]. The aim of the present work is to investigate the wear resistance of boride coatings produced on iron and steel by pack cementation and tested under both sliding and abrasive conditions, with particular regard to the influence of the crystallographic orientation of iron borides on the wear rate. 2 Experimental details Materials and treatments Sheets of Armco iron ( wt.% pure) and a medium carbon steel (UNI 38 NiCrMo 4) were annealed at 1000 ◦C under vacuum, surface finished with a 600 grit SiC emery paper and then borided at 850 ◦C for 15 h using a powder mixture constituted by B4C (20 wt.%), KBF4 (10%) and SiC (balance). Pure iron was selected in order to investigate boride coating free from alloying elements diffused from the substrate. The position of the boronising medium is suitable to grow polyphase coatings, constituted by an inner layer of Fe2B and an outer layer of FeB. The borided samples were characterised by means of optical (OM) and scanning electron microscopes (SEM), Xray diffraction analysis (XRD) and microhardness measurements (MHV). The XRD analyses were performed using a putercontrolled goniometer and the Co K_ radiation. The crystallographic texture of iron borides was evaluated at different depths from the external surface by gradually thinning the coating with the layerbylayer removal technique. The microhardness measurements were carried out through the thickness of the coatings on crosssections prepared with the usual metallographic techniques, using a conventional Vickers indenter and an applied load of N. For parative testing under sliding conditions, the medium carbon steel was also gas nitrided at 570 ◦C (total treating depth ∼ mm) or coated with a ∼10_m thick layer of hard chromium (hardness ∼67–68 HRC). An M 35 tool steel (position C wt.%, Cr wt.%, Mo wt.%, W wt.%, V wt.%, Co wt.%) coated with a layer of hard metal (WC–18%Co, hardness ∼88 HRA) deposited by the air plasma spray (APS) technique was selected as a reference, highly wear resistant material. Tribological tests Dry sliding tests were carried out using a puter controlled slideroncylinder tribometer (Fig. 1). The stationary sliders were constituted by the material under investigation, in the form of prismatic bars (5mm5mm50 mm). The counterfacing material was a ceramic coating consisting of Al2O3 (87 wt.%) and TiO2 (Rockwell hardness HRD = 60, surface roughness Ra = ) deposited onto the rotating cylinder. The tests were carried out under applied loads of 5 and 25N and sliding speed of −1, for sliding distances up to 5 km, at room temperature (20–25 ◦C), in laboratory air (relative humidity in the range 50–60%). Both friction resistance and system wear (. cumulative wear of both slider and cylinder) were continuously measured by means of a bending load cell and a displacement transducer, respectively, and were recorded as a function of the sliding distance. At the end of each test, wear scar depths were measured on both slider and cylinder by means of a stylus profilometer (pickup curvature radius, 5 _m), recording line profiles perpendicularly to the wear scar. The resistance of the boride coatings to abrasive wear was evaluated using a microscale abrasion tester (MSAT), which is based on a ballcratering geometry [8,9]. The rotation of a sphere against a flat specimen in the presence of small abrasive particles generates a wear crater with an imposed spherical geometry within the material. Basically the rig consists, as shown in Fig. 2, of a hard martensitic steel sphere (radius R = mm, hardness HV 1000) rotating against the specimen under investigation in presence of an abrasive slurry (an aqueous suspension of SiC particles 4–5 _m in size, with an initial concentration of g cm−3), maintained and replenished at the contact region by a slow constant drip feed (∼ cm3 min−1). A contact load of was used and the sliding speed was −1. The diameter b of the spherical cap produced on the specimen by abrasion was measured with a calibrated optical microscope, and the value of b was used to calculate both the peration depth h and the wear volume V: h ≈ b2/8R (1) V ≈ πb4/64R (2) where b ＜＜ R. If the depth of wear craters is lower than the thickness of the coating, a simple model for abrasive wear of bulk materials (equivalent to t。