Thermal spray coatings

The non-destructive characterization of thermal spray coatings for thermal barriers, anti-wear or anti- corrosion coatings is of particular importance for quality control, but it can be also very useful for understanding the mechanical and thermo-mechanical behavior of parts allowing the development of life prediction models. Laser-ultrasonics, a non-contact technique with great flexibility for generation and detection spot geometry, presents many advantages over conventional ultrasonic techniques for this application. In this paper, some recent works on the characterization of thermal sprayed coatings by laser- ultrasonics are presented. First, the use of surface acoustic waves to obtain elastic properties, density and thickness of dense coatings is considered and results obtained with WC-Co coatings are presented. Second, elastic property measurements on thermal barrier coatings are introduced and, finally, results on the detection of defects like vertical cracks and coating disbonding for dense coatings are discussed. Keywords: Laser ultrasound, coating, thermal spray, WC-Co, surface acoustic waves

Therefore, a need exists to examine thermal spray coatings that could enable the implementation of coated aluminum brake rotors. This paper presents the coating development work, including characterization and performance assessment, to assess the viability of a light weight brake rotor disc for applications in cars and light trucks. Arc-sprayed, cold-sprayed and hybrid-sprayed coatings based upon 300-series stainless steel alloys were produced and characterized in terms of adhesion, microstructure and hardness. The performances of these coatings under wear, thermal cycling and in corrosive environment were assessed at the lab scale. Finally, the coating determined to have the best performance was tested on a lab-scale dynamometer using a full-size prototype disc brake.

An alternative to SAW measurements is bulk wave measurements. Even though the experimental setup could be simpler, through thickness bulk waves measurements do not allow measuring all the above properties of the coating since at best one gets only two experimental values (i.e. the longitudinal and shear time-of-flights or resonance frequencies). To overcome this limitation, two quantities (i. e. density and one velocity) could be fixed, but in practice this is not reliable since these parameters are very dependant upon the thermal spray process. Further, we verified that it was even difficult to get the measurements. Due to the laser impact disturbance, the transmission in the substrate and the attenuation in the coating, no shear resonance frequency was actually observed and the signal-to-noise ratio for the longitudinal resonance frequencies was poor. As a result, SAW measurement is more precise and provides a more complete characterization of dense thermal spray coatings.

Abstract This paper reports on the performance evaluation of stainless steel (SS) thermal spray coatings aimed at shielding lightweight aluminum (Al) brake rotor disks from excessive heat and providing an adequate tribological surface in contact with brake pads. Coating wear, corrosion and heat resistance performances were evaluated using pin-on-disk, cyclic corrosion tests and thermal cycling using a custom laser rig, respectively. Arc spray optimized coatings displayed lower or equivalent wear rates when compared with the baseline gray cast iron disks, with similar frictional behavior. However, arc spray coating exhibited low adhesion which limits the maximum coating thicknesses achievable and leads to early coating spalling after about 1000 thermal cycles. Arc sprayed coatings also corroded and delaminated under corrosion tests. Optimized cold spray coatings present high corrosion resistance and could resist above 10,000 thermal cycles without spalling. However, cold spray coatings exhibit wear rates at least 4 times those of the cast iron. Taking advantage of both types of coatings, it was found that the production of a duplex coating made of a cold spray bond coat and an arc spray top coat could meet the requirements for protecting Al disks, with near 50% weight reduction.

indicates a shift in the residual stress state within the coupons. While the sub-surface initiations observed in the shot peened coupons are indicative of residual compressive stresses, the near surface initiation sites indicate that these pre-existing stresses were relieved during the coating process. Previous reports have proposed that heat flux resulting from the coating process may have a significant effect on the fatigue resistance of forged or heat treated substrates, particularly in cases where shot peening treatments have been applied prior to coating [5,20,21]. In the present study, the micrographs in Figure 9 show no noticeable evolution in the grain size or structure. The precipitates in the grains remain randomly dispersed and are not likely affected by the thermal spray process on to the coated fatigue samples with respect to the untreated 7075 Al alloy shown in Figure 8. It is important to note that arc spraying propelled liquid metal to the surface without the use of a flame that would add further heat into the substrate. Consequently, after arc spraying, the bulk temperature of the fatigue sample reached a temperature of about 65 ∞C which is far below the 150 to 300 ∞C reached EFFECT OF DUCTILE THERMAL SPRAY COATINGS ON FATIGUE PERFORMANCE OF 7075-T651 ALUMINUM ALLOY 501

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One factor that can affect the behaviour of coatings deposited by thermal spray is the presence of residual stresses that develop during the deposition and cooling process. An investigation of the residual stresses in these coatings has not been a focus of this study; however, preliminary work on this aspect has been performed using Almen strips 12 to evaluate the nature of the residual stresses present in some of the coatings used to produce the results shown in Figs. 4–6. Of the six coatings included in Fig. 4, only the four coatings deposited by APS and HVOF were evaluated to determine the stress state. This approach does not provide absolute stress values but it does indicate whether the coatings are under tension or compression. The measurements indicated that the two coatings (APS and HVOF) produced using the conventional powder were both in compression. For the coatings deposited using the nanostructured material, the stress states were compressive and neutral for the HVOF and APS coatings respectively. In comparing the degree of curvature for the various Almen strips, no correlation could be found between the stress state of the coatings and the performance in the abrasion tests or the extent of cracking under an indenter. Therefore, although residual stress can affect these values, it is believed that this factor is not playing a dominant role in the performance of these materials.

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Description THe present investigation has been conducted to study the tribological properties of Al 2 O 3 - 13pctTiO 2 (AT-13) ceramic coatings deposited on a low carbon steel type E335 by using a thermal flame spray technique. The microstructure and phase composition of wire and coatings were analyzed by scanning electron microscope, energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). Measurements of micro hardness were also performed on the surface of the coatings. The tribological tests were carried out using a pin-on-disk tribometer at different loads. All tests were performed using two disks as counter body, namely Al 2 O 3 -ZrO 2 (AZ-25) and Al 2 O 3 -TiO 2 (AT-3) which formed couple 1

All tests were performed using two disks as counter body, namely Al2O3-ZrO2 (AZ-25) and Al2O3-TiO2 (AT-3) which formed couple 1 and couple 2, respectively, in order to work out the wea[r]

Lately, different research results were reported for the production of cermets by plasma spraying, since it was considered to be the most flexible coating technique among thermal spray technologies [8 –11] . Despite chromium carbide's excellent wear- and corrosion-resistant properties, it is not used as primary carbide in industry owing to its lower hardness in comparison to other carbides (such as tungsten car- bide). Therefore, several attempts have been made to improve the hard- ness, wear property and corrosion of chromium carbide coatings by varying the coating techniques, processing parameters, characteristics

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However, there are other events that need to be considered. Rangaswany et al. [ 26 ] measured the CTE of APS oxide ceramics, from RT up to 1000 ◦ C. Two types of samples were analyzed: (i) free-standing coatings and (ii) coatings attached to steel substrate. They reported that the CTE values of the free-standing ceramic APS coatings were in the range of those of bulk ceramics, whereas those of the coatings attached to steel substrate nearly matched that of the steel substrate. This near-match proceeded probably because at high temperatures, the splats followed the expansion of the substrate. As the lamellar microstructure of thermal spray coatings allow the splats to slide and adjust over each other, they tend to exhibit “good” strain tolerance thereby accommodating CTE mismatch effects. According to Yanar et al. [ 27 ], the CTE values of Rene’ N5 (similar to Rene’ N515 employed in this work) at ~840 ◦ C and ~1030 ◦ C are ~14.5 and ~15.7 × 10 −6 / ◦ C, while that of APS YSZ from 800 to 1200 ◦ C is nearly constant at ~11 × 10 −6 / ◦ C. Therefore, the CTE of the metallic substrate is on average ~37% higher than that of the APS YSZ within the respective temperature range. For this reason, it is hypothesized that the higher CTE of the metallic substrate pulled the splats apart during the thermal gradient testing, thus slowing down sintering and densification developments.

velocities needed to produce shear localization for Cu on Cu (570-585m/s) vs Cu on 316L stainless steel (570-580m/s). Even though these velocities were easily achieved in the current work with N 2 as the process gas (i.e., when the He bond coat was not used), coatings would spontaneously debond from polished steel substrates before reaching a thickness of 1 mm. The reason for this behavior is that the critical velocities calculated above corresponded to the velocity for which shear instability was initiated and is insufficient to predict the extent of particle/substrate deformation needed to provide sufficient bonding. Figure 11 (a) shows the steel surface after spontaneous coating debonding. The surface is deformed due to particle impacts but a very low fraction of it displays traces of Cu, indicative of metallurgical bonding. The extent of adiabatic shearing will govern the fraction of the interface that is bonded, and would be a better indication of coating adhesion. For instance, it has been shown by Assadi et al. [33] that bond strength of Cu coating on Cu was about 30-40 MPa, which corresponds to 20% of the UTS of bulk Cu, for spraying conditions where the particles in-flight velocities were just above the critical velocity. This showed good agreement with their modelling work estimating that 15 to 25% of the interfacial area underwent shear instability.

zirconia-rich shell. Only particles larger than 2 lm were successfully cleaved due to limitations in the polishing media. The core of the particles appears porous or even hollow in some cases. During evaporation of the solvent, the ceramic concentration builds up near the droplet surface to form a concentrated ceramic shell as the droplet shrinks. This layer may inhibit further diffusion of the solute (ethanol) and delay complete melting. Ozturk et al. describe a similar mechanism in liquid precursor thermal spraying and delineate the conditions leading to hollow sphere particles (Ref 36). A unique feature here is the segregation of the ceramic within a droplet into the zirconia-rich shell and alumina-rich core. It can be spec- ulated that the segregation is caused by a higher affinity of zirconia to the solvent (over alumina), allowing the solvent to entrain the zirconia particles as it flows from the center to the outside of the droplet during evaporation. The melting and mixing of the molten ceramics is limited

Ni-WC/Ni Coatings For the Ni-WC/Ni coatings, Table 3 summarizes the WC fraction, overall DE, partial DE of Ni and WC, and porosity. Figure 5 shows micrographs of cross-sectioned Ni-WC/Ni composite coatings sprayed using various WC/ Ni contents in the feedstock. Using WC/Ni composite powder, the retention of WC in the coating was signifi- cantly improved and the WC content was close to that of the initial feedstock composition. This is related to the porous and agglomerated structure of the powder, which allows particle densification and deformation during deposition (Ref 13 - 17 ). Voids present between aggregated WC particles in the WC/Ni powder, along with porosity inside the powders and Ni binder between WC agglomer- ates, provide ductility. Upon impact at high velocity, compaction and deformation of WC/Ni particles near the contact area occurred through slipping and rotation of WC particles along the Ni binder. This is referred to as the pseudo-deformation of the particles (Ref 13 ). As WC/Ni content in the initial feedstock increased, the overall DE decreased. With 95 vol.% WC/Ni added to the initial Ni powder (76 vol.% WC in total), a thin coating with lateral cracks along lamellar interfaces was obtained (Fig. 5 d). When using WC/Ni powders as feedstock, with no Ni addition, the DE was very low and only a few of highly deformed and flattened particles with lateral cracks were deposited onto the substrate (Fig. 5 e and f).