Ceramic Matrix Composite Coatings Research Articles

Effect of Cobalt Fraction Mixing WC Clads on Microstructural Evolution, Crack Formation and Tribological Properties by Laser Cladding

In the present work, a ceramic-matrix composite coatings with different weight percentages of Co additive in the WC powders was studied on 45 steel by laser cladding. The effect of cobalt mixture with WC on mechanical properties such as microstructural evolution, hardness, wear and residual stress behavior of the coatings was carried out. The results show that high residual stresses induce significant cracking on 100% WC and that cracking activity is reduced by adding concentrations of 20% Cobalt to WC. Further, the microhardness of the coatings was more than five times higher than that of the substrate. The observation of the microstructural evolution reveals the fracture mechanism is a typical intergranular fracture in the molten zone and the crack propagates through the interior of the unmelted WC at 100% WC. Overall, the additive of high cobalt in WC did not result in a reduction of residual stresses due to less amount of brittle carbide. However, a reduced number of microstructural defects as well as a better wear behavior were obtained for the WC coated layers with suitably added Cobalt alloys.

Effects of the chemistry of coating and substrate on the steam oxidation kinetics of environmental barrier coatings for ceramic matrix composites

Environmental barrier coatings (EBCs) are an enabler for SiC/SiC ceramic matrix composites (CMCs) in gas turbines by protecting CMCs from environmental degradation. A critical EBC failure mode is the EBC spallation due to a build-up of elastic strains caused by the formation of SiO2 scale, known as TGO (thermally grown oxide). H2O, a byproduct of combustion reactions, accelerates the TGO-induced EBC failure by increasing TGO growth rates by orders of magnitude. NASA’s approach to improve the EBC life, therefore, is to reduce TGO growth rates. NASA discovered that modifying the TGO chemistry by modifying the EBC chemistry of Gen 2 EBC (Si / Yb2Si2O7) reduces the TGO thickness by up to ∼80 %. A study was undertaken to understand the oxidation mechanism of modified Gen 2 EBCs as well as to investigate the effect of EBC and CMC chemistry on TGO growth rates. This study confirmed the previously proposed TGO-controlled oxidation mechanism of modified Gen 2 EBCs and determined the correlations between the EBC and CMC chemistry, TGO chemistry, and TGO growth rates.

Fretting wear behaviour and frictional force mapping of Al2O3 based thermal barrier coatings

Efficiency and structural stability of gas-turbine engines were acheived with compliant thermal barrier coatings (TBCs) on structural components. In TBCs, top ceramic coatings encountered with other structural parts in a relative motion (amplitude of 100–500 μm and frequency of 1–5 Hz) was leading to failure, namely fretting wear. So, a high hardness material, Al2O3 based ceramic matrix composite (CMC) coatings were utilized to improve the tribological properties of the engineered surface. At this juncture, fretting wear behaviour of atmospheric plasma sprayed Al2O3–3 and 8 mol% Y2O3 stabilized ZrO2 (3YSZ and 8YSZ)‑carbon nanotube (CNT) based CMC coatings were investigated. The fretting wear rate of Al2O3 based CMC coatings against WC ball was observed as 0.97–1.87 × 10−6 mm3N−1 m−1. The least in wear resistance of Al2O3-CNT (1.87 × 10−6 mm3N−1 m−1) among the composite coating is attributed to low hardness and high Hertzian contact pressure (1.65 GPa) on the coatings. Among the composite coatings, Al2O3-3YSZ and Al2O3-8YSZ-CNT showed an improved wear resistance (~40.1% and ~34.5%, respectively) with wear rate of 0.97 × 10−6 mm3N−1 m−1 and 1.06 × 10−6 mm3N−1 m−1, respectively. It is attributed to improved mechanical properties, altered surface by reciprocating motion and the presence of wear debris, which reduces the wear rate by acting as a third body. So, there is an overestimation of wear rate (4.1–6.6 times) compared to that of experimentally calculated wear rate. Further, the Al2O3-YSZ-CNT composite showed fatigue crack on the fretting wear surface due to low plasticity index (62.0–109.3 MPa) of the coating. Also, the gradual change of running-in regime to the steady-state regime is achieved in the CNT reinforced composite coatings due to high resistance by the coatings which possess commendable mechanical properties. The frictional force mapping during the reciprocating motion confirms the rapid and gradual transition of two different regimes in the composite coatings.

Tribological performance assessment of Al2O3-YSZ composite coatings deposited by hybrid powder-suspension plasma spraying

The advent of high-throughput plasma spray systems that allow axial feeding encourages the study of using liquid feedstock for various next-generation functional applications. The current study explores the benefit of such a plasma spray system to deposit hybrid powder-suspension Al2O3-YSZ ceramic matrix composite (CMC) coatings for tribological applications. The tribological performance of the hybrid processed CMC coatings was assessed using scratch, ball-on-plate wear and erosion tests and compared with that of monolithic powder-derived Al2O3 coatings. As-deposited and tribo-tested coatings were characterized using Scanning Electron Microscopy, X-ray Diffraction and Energy Dispersive Spectroscopy to analyse their microstructure and phase constitution. The results showed that the tribological performance of the hybrid powder-suspension Al2O3-YSZ CMC coating was significantly improved by enhancing the wear resistance under scratch, dry sliding ball-on-plate and erosion tests as compared to the conventional APS deposited monolithic Al2O3 coating. About 36% decrease in the dry sliding ball-on-plate specific wear rate and up to 50% decrease in the erosion wear rate was noted in the hybrid powder-suspension Al2O3-YSZ CMC coating as compared to the conventional APS deposited monolithic Al2O3 coating. The study concludes that the hybrid powder-suspension route can create CMC coatings with unique multi-length scale microstructures which can be attractive for combining different tribological attributes in the same coating system.

Development of oxide-based High temperature environmental barrier coatings for ceramic matrix composites via the slurry process

A slurry process was employed to develop advanced environmental barrier coatings (EBCs) for SiC based composites. Yb2Si2O7 and mullite (3 Al2O3⋅2 SiO2) were chosen as the base materials for the bond coat to enable temperatures beyond the melting point of the current silicon bond coat (1414 °C). Various sintering aids, such as mullite, Yb2Si2O7, Al2O3, and silicon enabled sintering at 1500 °C – 1550 °C. Yb2Si2O7 was used as the top coat to provide water vapor recession resistance. A HfSiO4 intermediate layer prevented the 1500 °C eutectic reaction at the mullite/Yb2Si2O7 interface. Parabolic oxidation behavior and good adhesion as well as chemical and microstructural stability were observed after 500 h / 500 cycle steam oxidation at 1427 °C in 90 % H2O-10 % O2 steam. Kinetic analyses on Yb2Si2O7-coated SiC showed the independence of TGO growth rates on EBC thickness indicating that the oxidant permeation through the silica scale is the rate controlling factor.

Thermal Stress Analysis of Environmental Barrier Coatings Considering Interfacial Roughness

A numerical analysis of the effect of roughness interface on the thermal stress in the environmental barrier coatings for ceramic matrix composites was performed. Based on the concept of representative volume elements, a micromechanical finite element model of the coated composites was established. The rough interfaces between the coating layers were described using sine curves. The cooling process after preparation and the typical service conditions for the CMCs component were simulated, respectively. The results show that the rough interface has little effect on the temperature distribution along the depth direction for the studied T/EBC coatings for SiC/SiC composites. The stress concentration occurs at the rough EBC/BC interface, which is prone to cause delamination cracking. Under typical service conditions, the high temperature can eliminate part of the thermal residual stress. Meanwhile, the thermal gradient will cause large thermal stress in the TBC layer and the stress will result in surface cracks. The stress concentrations appear at the peaks and valleys of rough interfaces. The variation range of thermal stress increases with the roughness amplitude and decreases with the wavelength.

Development of Environmental Barrier and Interphase Coatings for Ceramic Matrix Composites

Development of Environmental Barrier and Interphase Coatings for Ceramic Matrix Composites

Effect of hBN and SiC addition on laser assisted processing of ceramic matrix composite coatings

To develop TiB2–TiN–SiC and TiB2–TiN–SiC-hBN based ceramic matrix composite coatings with superior mechanical and tribological properties suitable for engineering applications, a high-power CW-laser is used to initiate chemical reactions in precursor powder mixtures (TiO2, SiO2, hBN and graphite) of several compositions preplaced over AISI1025 steel substrates. Here, coatings’ stoichiometric compositions (TiB2–TiN–SiC) fabricated through in-situ synthesis are altered by adding SiC and additional amount hBN in the precursor at variable weight ratios. Presence of variable amount of SiC in TiB2–TiN–SiC and hBN in TiB2–TiN–SiC-hBN composites imparts several microstructural, mechanical and tribological property combinations. In this study, an attempt has been made to experimentally evaluate the optimum range of hBN and SiC additions to obtain highest improvements in lubrication and wear resistance respectively.

Thermodynamics equilibrium analysis on the chemical vapor deposition of HfC as coatings for ceramic matrix composites with HfClx(x = 2–4)-CyHz(CH4, C2H4 and C3H6)-H2-Ar system

A detailed thermodynamic equilibrium analysis in the chemical vapor deposition of HfC as coatings for ceramic matrix composites with HfClx(x = 2–4)-CyHz(CH4, C2H4 and C3H6)-H2-Ar system has been investigated using the FactSage code. To protect the excellent performance of the composites, we demand a suitable composition and yield of condensed phases. They have been examined as functions of the inject reactant ratios of HfClx/(HfClx + CyHz) and H2/(HfClx + CyHz), temperatures, and pressures. The results show that the HfCl4 species are the optimal Hf-source in the deposition process. The ideal ratio parameters for production of single phase HfC are Log[H2/(HfCl4 + CyHz)] = 2 and Hf/C = 1. And the optimum temperature condition is above 1200 K. A lower pressure of 5 kPa is found to be more favorable for the deposition of a pure HfC phase. The co-deposition of HfC with C or Hf will be easily obtained by only controlling the ratios of HfCl4/(HfCl4 + CyHz) and H2/(HfCl4 + CyHz). The HfCl4-C3H6-H2 precursor system should be the best choice in the experiment due to a higher yield of HfC at the same deposition conditions. The results in this work will be helpful for the further experiment investigation under different deposition conditions and put forward a new idea in exploration of the preparation method of coatings for ceramic matrix composites.

Evaluation of nanomechanical and tribological properties of laser surface alloyed boride-nitride-carbide ceramic matrix composite coatings

Interesting mechanical and tribological properties, favorable for both ambient and high temperature applications, can be gathered by composites consisting of multiple reinforcing phases. The present study focuses on the characterization of ceramic matrix composite coatings with in-situ reinforcements, developed through combination of laser triggered chemical reaction and subsequent laser treatment. A preplaced precursor mixture of TiO2, SiO2, hBN and graphite powders in stoichiometric proportions is used for the chemical reaction activated by a high power laser beam to produce coatings containing TiB2, TiN, SiC and hBN. Minor modifications in the constituents of the preplaced precursor mixture helped in developing coatings different in terms of the amount of hBN and SiC present in it. This compositional variation brought significant changes in mechanical and tribological properties of the coatings. X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) are used to confirm the presence of all the reaction products in the CMC coatings developed. Evaluation of nanomechanical properties and studies on the tribological properties of these coatings are performed thoroughly. Reduced modulus (Er) and nanohardness (H) of the composite coatings deposited on AISI 1025 steel have been measured from load–displacement curves. The stress-strain curves of composite coatings help in characterizing its damage tolerance and determining design stresses to find suitability to any application. Presence of hBN in composite coating causes reduction in coefficient of friction. Coatings formed with SiC in precursor helps in improving the specific wear rate. Top surface morphology and tribological properties of these coatings, after they are exposed to high temperature, are also explored.

Analysis of instrumented scratch hardness and fracture toughness properties of laser surface alloyed tribological coatings

The mechanical characteristics of ceramic matrix composite (CMC) coatings are widely different from the same materials in bulk form or the individual constituents and are very important to be assessed to carry out application oriented studies on CMC coatings with novel compositions. In the present work, a composite coating of TiB2, TiN and SiC is fabricated in-situ through a combination of high temperature chemical reaction and laser surface alloying. The formation of the surface layer is due to the laser-assisted chemical reaction followed by laser melting. A mixture of TiO2, SiO2, hBN and graphite in stoichiometric proportions is used as the precursor for the chemical reaction. The presence of all the reaction products in the CMC coatings developed is confirmed by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). A thorough evaluation of various mechanical properties achieved more insight into the CMC coatings developed. Hardness and fracture toughness of the coatings are measured with a scratch tester. The property evaluations are performed in a similar way for two more coatings fabricated with precursor mixtures containing more than a stoichiometric amount of SiC and hBN respectively. For comparison, a number of composites fabricated through various other routes are characterized afresh with the same set of techniques. Coatings formed with SiC in precursor show higher values of scratch hardness (14.37GPa), microhardness (24.37GPa) and fracture toughness (6.63MPa-m1/2).

Structure and properties of nanostructured ceramic matrix composite coatings prepared in-situ by reactive plasma spraying micro-sized Al–Fe2O3–Cr2O3 powders

Nanostructured FeAl2O4-based ceramic matrix composite coatings were prepared in-situ by reactive plasma spraying micro-sized Al–Fe2O3 and Al–Fe2O3–Cr2O3 powders. The microstructure, toughness, Vickers hardness, and adhesive strength of these coatings were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and mechanical tests. The results indicated that both the coatings exhibited a nanostructured microstructure. The grains of coating AFC sprayed with Al–Fe2O3–Cr2O3 powders are finer than those of the coating AF sprayed with Al–Fe2O3 powders. The composite nano-coating sprayed with Al–Fe2O3–Cr2O3 powders exhibited higher hardness and better wear resistance compared with those of the composite nano-coating sprayed with Al–Fe2O3 powders. The adhesive strength, toughness, and wear resistance of the composite coating sprayed with Al–Fe2O3–Cr2O3 powders were significantly enhanced compared with those of the composite coating sprayed with Al–Fe2O3 powders, which were attributed to the Cr2O3 addition.

Microstructure and tribological properties of laser clad CaF 2/Al 2O 3 self-lubrication wear-resistant ceramic matrix composite coatings

Self-lubrication wear-resistant CaF 2/Al 2O 3 ceramic matrix composite coatings were fabricated on substrates of Al 2O 3 by laser cladding CaF 2–Al 2O 3 mixed powder blends. Compared with laser clad monolithic Al 2O 3 coatings, the CaF 2/Al 2O 3 coating has much superior wear resistance and noticeably lower friction coefficient under dry sliding wear test conditions.