Thermal Shock Resistance Of Coatings Research Articles

Excellent thermal shock resistance of NiCrAlY coatings on copper substrate via laser cladding

• NiCrAlY layers were prepared on copper substrate via laser cladding technology. • Metallurgical bonding and excellent thermal shock resistance are obtained. • Lifetime of this NiCrAlY layers is much longer than NiCrAlY by other methods. Coatings are widely used to protect substrates in extreme thermal environments (e.g., arc heaters), and thermal shock resistance is a crucial parameter for the coatings, which requires tight interlayer bonding between coatings and substrates. In this work, NiCrAlY coatings were highly required for the pure copper substrate to restrict the electric arc in arc heaters. To overcome the bonding difficulty of coating on the copper surface, the NiCrAlY coatings were prepared by two laser cladding methods: conventional laser cladding (CLC) and high-speed laser cladding (HSLC). The microstructure, composition, and thermal shock resistance of NiCrAlY cladding layers prepared by both methods were investigated. Benefitting from the high cooling rate and high energy density, the HSLC-layer has better composition uniformity and tighter interlayer bonding than the CLC-layer, achieving a 30%–45% improvement in thermal cycling lifetime. Besides, the NiCrAlY layers prepared on copper substrate by both laser cladding methods exhibit 3–10 times better thermal shock resistance than those NiCrAlY layers prepared by conventional spraying methods. It further confirms the great effects of metallurgical bonding and composition uniformity on the thermal shock resistance of coatings. The NiCrAlY layers fail in the form of internal cracking and interface peeling, and the corresponding failure mechanism is discussed.

Microstructure and thermal shock resistance of Nd2O3-doped YSZ-based thermal barrier coatings

To study the impact of rare earth oxide doping on the thermal failure of thermal barrier coatings, 0.5 mol%, 1.0 mol% and 1.5 mol% Nd2O3-doped YSZ coatings were prepared by explosive spraying. SEM, XRD, EDS and microhardness testing were used to analyse the effect of different rare earth oxide doping contents on the morphology, composition and mechanical properties of the coatings. With an increase in the Nd2O3 doping content, the porosity of the coatings was reduced. The decrease in the porosity increased the compactness of the coatings and improved the microhardness and fracture toughness. The bonding strength and thermal shock resistance of the coatings were the highest among the samples herein when the rare earth doping content was 1.0 mol%, and the values were 37.6 MPa and 200 times, respectively. The thermal shock failure mode of the coating was mainly due to the exfoliation of the inner layer of the ceramic layer. The luminous intensity of the coating increased with increasing rare earth oxide doping content, and the emission spectrum of the Nd2O3-modified YSZ coating after the thermal shock test produced a new emission peak at 594 nm, which decreased at 708 nm.

Microstructure and thermal shock resistance of AlBOw- and BNw-whisker-modified thermal barrier coatings

To improve the crack propagation resistance of YSZ thermal barrier coatings during the thermal cycle, three kinds of thermal barrier coatings were prepared by atmospheric plasma spraying: YSZ, AlBOw-modified YSZ and BNW-modified YSZ. SEM, EDS and XRD were used to analyse the morphology, composition and phase composition of the sprayed powder and coating section. The phase structures of the YSZ, YSZ+AlBOw and YSZ+BNw coatings were t' phase. The cross-section of the coating presents a layered structure with pores inside. The porosity values of the YSZ, YSZ+AlBOw and YSZ+BNw coatings are 10.33%, 14.17% and 12.52%, respectively. The thermal shock resistance of three groups of coatings after 5 min at 1000 °C was analysed. The failure behaviour of the coatings after several thermal cycles was studied. The results show that the thermal shock resistance of the coatings with AlBOw is slightly lower than that of the YSZ coatings. The thermal shock resistance of the BNw coatings is 62.2% higher than that of the YSZ coatings. The whisker inhibits the crack propagation and prolongs the life of the coatings via crack deflection, whisker pull-out and whisker bridging.

Impact of cathode loss on plasma characteristics, microstructures and properties of 7YSZ coatings in PS-PVD

The slight changes in geometry and mass of the cathode of O3CP plasma torch during service life caused the variety of arc motion between the cathode and anode. The voltage appeared to rise first and then fall. In this experiment, the impact of cathode loss on plasma characteristics, microstructures and properties of 7YSZ coatings was investigated. Optical emission spectroscopy (OES) was used to characterize the plasma jet characteristics such as plasma temperature and electron number density. It is found that vapor deposition is the main deposition mechanism, and the microstructures of the coatings are mainly affected by the supersaturation and subcooling of the vapor phase, which determine the nucleation and growth of crystals. At the middle of cathode service life, the plasma temperature and electron number density are the highest and the powder is completely evaporated. High crystal growth rates and surface diffusion lead to large columnar crystals with large grain size. The best thermal shock resistance. The thermal shock resistance is the best in this case. When the cathode is in the beginning and end of service life, there are columnar grains with small size produced and many solid particles in the gaps due to diffusion difficulties generated by low temperature. Shadowing enhances to manufacture fine grains and few branches. The presence of solid particles prevents the release of thermal stress and reduces thermal shock resistance of coatings. Fine grains prevent cracks propagation to improve the bonding strength of coatings.

Mechanical Properties and Thermal Shock Resistance of 8YSZ-Al2O3 Composite Coatings with Different Thicknesses

The mechanical properties and thermal shock resistance of plasma-sprayed 8YSZ-Al2O3 composite coatings with different thicknesses have been analyzed. At a thickness of 330 μm, the Young’s modulus of the coating surface increased from 96.54 to 121.39 GPa and the surface hardness from 6.63 to 9.14 GPa when the Al2O3 content was varied from 10 to 40 wt.%. Adding more alumina in the coating resulted in a change in the coating surface residual stress from tensile (48.8 MPa) to compressive (−115.9 MPa), while the thermal shock resistance at 1100 °C decreased from 162 to 56 cycles. Moreover, at given Al2O3 content, the thermal shock resistance of the coating decreased drastically when its thickness was increased, which can be attributed to the enhanced Young’s modulus and hardness near the bond coat/ceramic coating interface. An increase in the ceramic thickness resulted in a significant stress gradient as well as strain energy in the direction of the coating thickness, bringing about high thermal stress during thermal shock testing. The horizontal crack propagation caused by thermal stress in the interface area could be the main reason for coating failure.

Influences of Laser Remelting on Microstructure and Thermal Shock Resistance of Al2O3-13wt.%TiO2 Coatings Deposited by Plasma Spraying

Al2O3-13wt.%TiO2 composite ceramic coatings were prepared by plasma spraying and laser remelting. Morphology, microstructure and phase composition were analyzed by scanning electron microscope (SEM) and X-ray diffractometer (XRD). Meanwhile, effects of laser remelting on thermal shock resistance of plasma-sprayed coatings were investigated and the thermal shock failure mechanism of coatings were discussed. Results demonstrate that after laser remelting, particles on the ceramic coating are refined and the lamellar structure disappears, thus increasing density and getting a remelting coating basically without defects like cracks. Laser remelting metastable phase γ-Al2O3 is transformed into a stable phase α-Al2O3. Due to low heat conductivity coefficient of ceramic materials, it is impossible to remelt the whole ceramic coating at laser remelting. An isometric crystal remelted zone with small grains, a sintered zone and a lamellar remained plasma-sprayed zone are formed on the remelting ceramic coating. Compared with plasma-sprayed coating, the laser-remelted coating has better thermal shock resistance. The thermal shock failure form of ceramic coating deposited by plasma spraying is basically corner peeling, while the thermal shock failure forms of laser-remelted ceramic coating includes both corner peeling and considerable local peelings in the center. Influences of laser remelting on thermal shock resistance of coating are mainly manifested as reduced initial failure resistance of coatings, decelerated crack propagation on coatings and changing failure mode of coatings.

Thermal shock resistance and thermal conductivity of diamond-Cu composite coatings on Cu substrate via mechanical milling method

In this work, diamond-Cu composite coatings were successfully fabricated on T2 pure Cu substrate by mechanical milling (MM) method. Effects of raw diamond particle size, diamond content and milling time on the microstructure and properties of the coatings were investigated. The results showed that the diamond particle size in the coating was mainly distributed between hundreds of nanometers and several micrometers. The microhardness of the coating was much higher than that of the Cu substrate. Larger raw diamonds would contribute to the thermal shock resistance of the coating, due to the interfacial strengthening of diamond inlay structure and the buffering effect of inner layer. The coating with optimum microstructure and thermal shock resistance could be synthesized at the raw diamond dimension of 105 μm, the diamond content of 30 wt% and the milling duration of 7 h. The formation mechanism of the coating was discussed. The thermal diffusion was closely related to the crystal structure of copper and interfacial bonding between diamond and copper. The thermal conductivity of the coated sample improved to 336.938 W m−1 K−1 after subsequent annealing treatment, even higher than that of T2 pure copper substrate.

Mullite whisker toughened mullite coating to enhance the thermal shock resistance of SiC pre-coated carbon/carbon composites

In order to improve the thermal shock resistance of the coated carbon/carbon (C/C) composites, a mullite whisker toughened mullite coating was fabricated on the surface of SiC pre-coated C/C composites (SiC-C/C) by molten-salt method with a later hot dipping process. The phase compositions, surface and cross-section microstructures, high temperature thermal shock resistance of the as-prepared multi-layer coatings were investigated. Results show that the introduction of mullite whiskers can effectively improve the density of the mullite outer coating and decrease the cracking of the coating during the thermal shock cycle process. After 100 times thermal shock cycles between 1773K and room temperature, only 1.87 × 10−3gcm−2 weight loss has been detected, indicating the achievement of the excellent thermal shock resistance.

Thermal shock resistance of bimodal structured thermal barrier coatings by atmospheric plasma spraying using nanostructured partially stabilized zirconia

In this article, an agglomerated and sintered Y2O3-stabilized ZrO2 (YSZ) powder was introduced as a feedstock for atmospheric plasma spraying, and bimodal structured thermal barrier coatings with different contents of nanostructure were deposited. Water-quenching method was carried out to study the effect of nanostructure content on thermal shock resistance of the coating. Results showed that the thermal cycle life of the coating is significantly influenced by the microstructure of the coating. When the nanostructure content was relatively low (about 39%), the coating showed high microhardness and poor thermal cycle life (failure at 11cycles); however, when the nanostructure content was relatively high (about 60%), the coating showed lower microhardness and improved thermal cycle life (failure at 42cycles); the coating with medium nanostructure (about 52%) showed the best thermal shock resistance (no failure at 50cycles). The thermal stability of nanostructured coating was also investigated. After heat treatment at 1300°C for 10h, the nanoparticle size increased and the hardness of the coating increased noticeably, indicating the sintering effect of the nanostructured coating.

Effect of Bond Coats on Thermal Shock Resistance of Thermal Barrier Coatings Deposited Onto Polymer Matrix Composites Via Air Plasma Spray Process

Thermal barrier coating systems with different bond coats were fabricated on polymer matrix composites via the air plasma spray process. During a thermal shock test at 400 °C, Zn and Al interlayers were helpful in improving the thermal shock resistance of coatings due to the low melting point. The coating system consisted of a soft zinc layer as a bond coat, and YSZ as a top coat exhibited the best thermal shock resistance, attributed to the lower residual stress and lower thermal stress in the Zn interlayer. The failure mechanism of the coating system was mainly ascribable to the residual stress derived from the deposition process, thermal stress, and further damage of the substrate.

Microstructure, bonding strength and thermal shock resistance of ceramic coatings on steels prepared by plasma electrolytic oxidation

Ceramic coatings were successfully prepared on steel by plasma electrolytic oxidation (PEO) in aluminate electrolyte and silicate electrolyte, respectively. The microstructure of the coatings including surface morphology, phase and element composition were studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The bonding strength between the ceramic coating and the substrate was tested using different methods including tensile tests and shearing tests. The thermal shock resistance of the coatings was also evaluated. The results indicated that coatings obtained in both electrolytes were porous and coarse. The average diameters of the pores were below 10 μm. PEO coatings obtained in aluminate electrolyte were composed of Fe 3O 4 and FeAl 2O 4, while those obtained in silicate electrolyte were in a noncrystal state. PEO coatings obtained in aluminate electrolyte showed similar change trend of tensile strength and shearing strength with increasing treating time, namely, a relatively high values with middle time treating and low value with short and long time treating. The best coating was the samples treated with 30 min, whose tensile strength was 20.6 MPa and shearing strength was 16 MPa. The tensile strength and shearing strength of coatings obtained in silicate electrolyte were not strongly influenced by the treating time, the values of which were range in 14 ± 2 MPa and 11 ± 2 MPa, respectively. Coatings obtained in both electrolytes showed the best thermal shock resistance with middle time treating. Coatings obtained in silicate electrolyte show a little better thermal shock resistance than those obtained in aluminate electrolyte.

Thermal shock resistance of ceramic coatings

A methodology is established for the evaluation of the thermal shock resistance of ceramic coatings. The problem is formulated by considering a thin coating to a thick substrate subjected to a sudden cooling on the coating surface. Time-varied thermal stresses and stress intensity factors for various parameters of the problem are obtained. The crack initiation behavior and the thermal shock resistance of the coating are studied. The crack length dependency behavior of the thermal shock resistance of the coating is discussed. Two critical size parameters, which control the applicability of the stress-based criterion and the fracture mechanics-based criterion to the determination of the thermal shock resistance of coatings, are explored.