Microstructural and tribo-mechanical properties of arc-sprayed CoCr-based coatings

Due to their superior wear and oxidation resistance, Stellite™ coatings are widely used in industrial applications, where the coatings are exposed to high temperature. Common processes for applying Stellite™ coatings include the high-velocity oxy-fuel spraying, laser cladding, and plasma transferred arc welding. Although Stellite™ welding consumables or similar welding consumables in the form of cored wires (CoCr base without industrial property rights) are commercially available, there are hardly any studies on arc-sprayed Stellite™ coatings available in the literature. In this study, the microstructural characteristics of arc-sprayed deposits were investigated, which were produced using a CoCr-based cored wire with addition of 4.5 wt.% tungsten. The produced deposits were examined in its as-sprayed state as well as after exposed to elevated temperatures. The microstructure was scrutinized by means of electron microscopy, energy-dispersive x-ray spectroscopy, as well as x-ray diffraction analyses using synchrotron radiation. Tribo-mechanical tests were conducted in order to assess the performance of the arc-sprayed coating. The findings were discussed and compared to those obtained from conventional CoCr-based coatings. It was found that the arc-sprayed CoCr-based coating is predominantly composed of Co-rich, Cr-rich lamellae or lamellae comprising a Co(Cr)-rich solid solution interspersed with various oxides between the individual lamellae. Solid solution hardening serves as dominant strengthening mechanism, while precipitation hardening effects are hardly evident. With regard to the oxidation behaviour, the as-sprayed coating mainly contains CoCr2O4 as well as traces of Co3O4. For heating above 550 °C, coating surface additionally consists of Fe2O3 and Co3O4. In dry sliding experiments, the arc-sprayed CoCr-based coating shows a decreased wear resistance compared to CoCr-based coatings processed by HVOF and PTA, whereas the coefficient of friction (COF) sliding against alumina was similar to the COF observed for the HVOF-sprayed CoCr-based coating, but lower than the COF obtained for the CoCr-based hardfacing alloy deposited by PTA

WC decomposition phenomena in ID-HVOF-sprayed WC-CoCr coatings using fine powder feedstock

Over the last few decades, the high velocity oxygen fuel (HVOF) spraying of WC-CoCr for internal diameter (ID) coating has attracted much interest for hard chrome replacement. Current demands for the ID coating of small cylindrical parts necessitates the use of specialized spray gun equipment and powder feedstocks with small particle size fractions. Due to the limited spray distance inside cylindrical parts with small IDs, the process control, spraying fine WC-CoCr powders, meets new challenges to avoid significant WC decomposition, which increases the risk of mechanical degradation. Within the scope of this study, ID-HVOF spraying using a fine-structured WC-CoCr (−15 + 5 μm) feedstock with a mean WC particle size of 400 nm is examined with respect to the WC decomposition phenomena using X-ray diffraction (XRD). Hence, a statistical design of experiments (DoE) is utilized to systematically analyze various spray parameter settings along with their interaction as part of the WC to W2C conversion

Embedding behavior of ceramic particles in babbitt coatings and its effect on the tribological properties of low-pressure cold sprayed coatings

The low melting point of Sn-based Babbitt alloys often causes nozzle clogging in the low-pressure cold gas-dynamic spraying (LPCGDS) process, which impacts the process steadiness and the coating quality. Adding hard particles to the feedstock material eliminates this kind of interruption. A certain amount of these particles finds their way in the obtained coatings. These particles also trigger a kind of “hammering effect” due to their impulse forces. These forces are directly dependent on the mass and velocity of the impacting hard particles. However, these forces may lead to a decrease in the porosity and improve the adhesion of the obtained coating. In this study, the effect of the density and size of the hard particle was examined by three different hard materials, Cr3C2, Al2O3, and B4C, which have a material density of 6.68, 3.95, and 2.52 g/cm3, respectively. The used feedstock in this study is a powder mixture that contains 75 vol.% Babbitt and 25 vol.% of either B4C, Cr3C2, or Al2O3. The effect of the size distributions “particles with lower mass” was tested using two different Al2O3. The various hard particles show different embedding behaviors, as well as different effects on the coating build-up. It was found that the blended hard particles were enclosed with the Babbitt matrix, and their interface with Babbitt shows no clear evidence of pronounced diffusion. The size distribution of the blended hard particles has a direct effect on the splat formation and the obtained coating microstructure. It was found that the type of hard particles played a decisive role in the friction behavior. Nevertheless, the hard particle reinforced Sn-Sb-Cu-based composite coatings demonstrated a nearly constant coeffcient of friction throughout the load-interval

Microstructural characteristics in Babbitt coatings deposited by LPCS

Studies have already established that the mechanical properties of Babbitt coatings significantly depend on the microstructural characteristics, such as the amount and distribution of intermetallic compounds dispersed in a soft solid solution matrix. For Sn–Sb–Cu-based Babbitt coatings, the formation of SbSn- and CuSn-based precipitates has a substantial influence on the resulting microhardness and thus determines the maximum load carrying capacity. Thermal spraying of Sn-based Babbitt coatings results in a relatively more refined structure of these precipitates than in common manufacturing processes, such as casting, due to the thermal processing conditions. This study aims to evaluate the effect of the temperature of the propellant gas and substrate temperature on the microstructural characteristics of Sn–Sb–Cu-based Babbitt coatings deposited by low pressure cold spraying (LPCS). The deposits were examined for their phase composition, microhardness and mesoscopic structure. It was found that the coatings were mainly composed of Sb2Sn23, Sb0.49Sn0.51 and Sorosite (CuSn or CuSb0.115Sn0.835), regardless of the substrate temperature or temperature of the propellant gas to be investigated. For a gas temperature above 300 °C, an increased microhardness was observed, which correlates with the appearance of a more homogenous distribution of Sb0.49Sn0.51 dispersed in a soft Sn-rich solid solution matrix

Effect of substrate pre-treatment on the low cycle fatigue performance of tungsten carbide-cobalt coated additive manufactured 316 L substrates

Numerous studies already identified that the fatigue strength of 316 L parts processed by laser beam melting (LBM) is distinctly affected by the surface integrity. Among others, surface defects as well as residual stresses are of crucial importance. Despite new findings in the field of surface engineering of laser beam melting (LBM) parts, the low cycle fatigue strength of thermally sprayed additively manufactured substrates has not been in the focus of research to date. This study aims at evaluating the effect of different pre-treatments onto 316 L substrates processed by laser beam melting (LBM) prior to the deposition of a high velocity oxy-fuel (HVOF) sprayed tungsten carbide-cobalt coating and their effect on the low cycle fatigue strength. Therefore, 316 L substrates were examined in their as-built state as well as after grit blasting with regards to the surface roughness, strain hardening effects, and residual stresses. To differentiate between topographical effects and residual stress related phenomena, stress-relieved 316 L substrates served as reference throughout the investigations. The tungsten carbide-cobalt coated and differently pre-treated 316 L substrates were mechanically tested under quasi-static and dynamic load conditions. Besides the low cycle fatigue strength, the fracture toughness as well as the fracture mechanism were identified based on fracture surface analysis

Change Detection Using Landsat and Worldview Images

This paper presents some preliminary results using Landsat and Worldview images for change detection. The studied area had some significant changes such as construction of buildings between May 2014 and October 2015. We investigated several simple, practical, and effective approaches to change detection. For Landsat images, we first performed pansharpening to enhance the resolution to 15 meters. We then performed a chronochrome covariance equalization between two images. The residual between the two equalized images was then analyzed using several simple algorithms such as direct subtraction and global Reed-Xiaoli (GRX) detector. Experimental results using actual Landsat images clearly demonstrated that the proposed methods are effective. For Worldview images, we used pansharpened images with only four bands for change detection. The performance of the aforementioned algorithms is comparable to that of a commercial package developed by Digital Globe

Metacognitive Therapy for Depression in Adults: A Waiting List Randomized Controlled Trial with Six Months Follow-Up

This randomized controlled trial examines the efficacy of metacognitive therapy (MCT) for depression. Thirty-nine patients with depression were randomly assigned to immediate MCT (10 sessions) or a 10-week wait list period (WL). The WL-group received 10 sessions of MCT after the waiting period. Two participants dropped out from WL and none dropped out of immediate MCT treatment. Participants receiving MCT improved significantly more than the WL group. Large controlled effect sizes were observed for both depressive (d = 2.51) and anxious symptoms (d = 1.92). Approximately 70–80% could be classified as recovered at post-treatment and 6 months follow-up following immediate MCT, whilst 5% of the WL patients recovered during the waiting period. The results suggest that MCT is a promising treatment for depression. Future controlled studies should compare MCT with other active treatments

Adhesion of HVOF-sprayed WC-Co coatings on 316L substrates processed by SLM

Different studies have been demonstrated that the surface integrity of substrate bulk materials to be coated has a significant impact on the adhesion of thermally sprayed coatings. It is known that the surface integrity of parts processed by selective laser melting (SLM) differs from those obtained from bulk materials. Although 316L stainless steel is among the most investigated material for SLM, the adhesion of thermally sprayed coatings on 316L stainless steel substrates processed by SLM has not been studied yet. This study aims at evaluating the effect of various mechanical pre-treatments onto 316L stainless steel substrates processed by SLM and their effect on the adhesion of high velocity oxy-fuel (HVOF)-sprayed WC-Co coatings. To differentiate between topographical effects and residual stress-related phenomena, a stress-relief heat treatment of the SLM substrates served as a reference throughout the investigations. The differently pre-treated SLM substrates were investigated with regard to the surface roughness and residual stresses. For the HVOF-sprayed SLM composites, Vickers interfacial indentation tests were conducted to assess the resulting coating adhesion. The findings demonstrated that the HVOF-sprayed WC-Co coatings predominantly exhibit good adhesion to the SLM 316L substrates. However, it was found that the stress state in the SLM 316L substrate surface is more likely to affect the adhesion of the WC-Co coating, while the substrate surface roughness showed a marginal effect

Temperature-induced formation of lubricous oxides in vanadium containing iron-based arc sprayed coatings

In the field of surface engineering, the use of self-lubricous coatings with the incorporation of vanadium represent a promising approach to reduce friction, thus contributing to the wear behavior. For vanadium containing hard coatings produced by means of thin film technology, the reduction in friction at elevated temperatures was repeatedly attributed to temperature-induced and tribo-oxidatively formed oxides which act as solid lubricant. Only very few studies focused on the tribological characteristics of vanadium containing arc sprayed coatings. In this study, the tribological characteristics of a vanadium containing iron-based arc sprayed deposit were investigated in dry sliding experiments under ambient conditions and different temperatures. Types of wear at the worn surfaces and counterparts were examined by means of electron microscopy and energy dispersive X-ray (EDX) spectroscopy. The speciation of vanadium in the superficial layer was determined using X-ray absorption near edge structure (XANES) spectroscopy. It was found that the vanadium-containing coating exhibited a distinctly reduction of the coefficient of friction above 450 °C which further decreased with increasing temperature. XANES spectroscopy indicated an increased oxidation state for the V component on the coating surface, suggesting the prevalence of specific vanadium oxides which promote a self-lubricating ability of the coating