Mechanical properties of ceramic thermal sprayed coatings

Recently, significant emphasis has been placed on the research of functional coatings for industrial components to cope with the demands of extreme operating conditions. One of these cutting-edge coating techniques is the thermal spray process, which a coating is created by the rapid solidification and interlocking of impinging molten droplets called 'splats'. Dr Ang’s research was to evaluate the mechanical response of thermal spray ceramic coatings. He showed that due to the deposition of the splats in a highly oriented fashion and the presence of the intrinsic void systems, thermal spray coatings showed anisotropic mechanical properties. Subsequently, various coating survivability models were constructed to predict the mechanical strength for respective orientations

A review of testing methods for thermal spray coatings

The primary focus of this review concerns the test methods used to evaluate thermal spray coatings. Techniques to measure coating intrinsic properties such as (i) porosity and (ii) residual stress state; as well as extrinsic mechanical properties that include (iii) hardness, (iv) adhesion, (v) elastic modulus, (vi) fracture toughness, and (vi) the Poisson’s ratio of thermal spray coatings are presented. This review also encompasses the feedstock and thermal spray method since process variants create a specific microstructure. An important aspect of this work is to highlight the extrinsic nature of mechanical property measurements with regard to thermal spray coatings. Thermal spray coatings exhibit anisotropic behaviour and microstructural artefacts such as porosity and the splat structure of coatings influence the mechanical characterisation methods. The analysis of coating data variability evolving from the different measurement techniques is of particular relevance to interpret the character of thermal spray deposits. Many materials can be thermal sprayed but this review focuses on alumina and partially stabilised zirconia since (i) these materials have many proven applications, and (ii) there is a wealth of information that has been reported on these ceramics

Investigating the anisotropic mechanical properties of plasma sprayed yttria-stabilised zirconia coatings

The adhesion and cohesion bond strengths of plasma sprayed yttria-stabilised zirconia (YSZ) coatings were measured by performing the tensile adhesion test (TAT) and the tubular coating tensile test (TCTT). The TAT allowed assessment of adhesive/cohesive bond strength of a coating microstructure perpendicular to the substrate. In contrast, the TCTT quantifies the strength of a thermal spray coating parallel to the substrate without the use of any adhesive. The failure strength of the coatings from the respective tests can be approximated to a Weibull distribution and indicated the anisotropic behaviour of plasma sprayed coatings. The average coating strength parallel to the substrate is approximately 1.5 times greater than the bond strength perpendicular to substrate. The anisotropic behaviour of the plasma sprayed YSZ coatings were also probed using Knoop hardness measurements that were orientated at a well-defined geometry with respect to the lamellar microstructure. In addition, uniaxial compression tests evaluated the Poisson's ratio of these anisotropic YSZ coatings when loaded with respect to the different microstructural orientations

Mechanical properties of plasma sprayed YSZ coatings measured using TAT and TCT test

The adhesion and cohesion bond strength of plasma sprayed YSZ coatings were measured by performing tensile adhesion test (TAT) and tubular coating tensile (TCT) test. The TAT allowed measurements of adhesion/ cohesive bond strength of YSZ-bond coat microstructure perpendicular to the spray direction. The TCT test was used to measure the strength of a thermal spray coating parallel to the spray direction without the use of any adhesive. The failure strength of the coatings from the respective tests can be approximated to a Weibull distribution and also indicated the anisotropic behaviour of YSZ plasma sprayed coatings. The average coating strength parallel to the spray direction is approximately 1.5 times greater than the bond strength perpendicular to spray direction. The anisotropic behaviour of YSZ coatings have also been verified by Knoop hardness measurements

Development of processing windows for HVOF carbide-based coatings

Optimized processing windows for spraying high-quality metal carbide-based coatings are developed using particle diagnostic technology. The cermet coatings were produced via the high-velocity oxygen fuel (HVOF) spray process and are proposed for service applications such as marine hydraulics. The traditional 'trial and error' method for developing coating process parameters is not technically robust, as well as being costly and time consuming. Instead, this contribution investigated the use of real-time monitoring of parameters associated with the HVOF flame jets and particles using in-flight particle diagnostics. Subsequently, coatings can be produced with knowledge concerning the molten particle size, temperature, and velocity profile. The analytical results allow identification of optimized coating process windows, which translate to coatings of lower porosity and improved mechanical performance

Modeling the coverage of splat areas arising from thermal spray processes

The coverage of a deposited material that arises from the integration of discrete splat areas is an essential parameter that needs to be understood during thermal spray processes. However, there is absence of a theoretical method to predict and estimate the area coverage per pass by a thermal spray torch; for example by the plasma spray process. In this study, a model is presented that calculates the splat area coverage for a thermal spray process of ceramic materials. A focused survey of the published literature takes into consideration experimental observations that are related to this work. The model accounts for physical events in thermal spray processes, such as the use of a mono-modal feedstock and size of spray stream. The model predictions for the area coverage of plasma sprayed yttria-stabilized zirconia (YSZ) ranged between 12% and 18% depending on the value assumed for the maximum flattening ratio. The results for the model were demonstrated via wipe studies where microscope glass slides were spray coated with YSZ and image analysis conducted. The average coverage calculated was approximately 13% for the sprayed images. The experiment verifies that the simulation results from the model predict adequately the splat area coverage of a thermal spray process

Thermal spray maps: material genomics of processing technologies

There is currently no method whereby material properties of thermal spray coatings may be predicted from fundamental processing inputs such as temperature-velocity correlations. The first step in such an important understanding would involve establishing a foundation that consolidates the thermal spray literature so that known relationships could be documented and any trends identified. This paper presents a method to classify and reorder thermal spray data so that relationships and correlations between competing processes and materials can be identified. Extensive data mining of published experimental work was performed to create thermal spray property-performance maps, known as "TS maps" in this work. Six TS maps will be presented. The maps are based on coating characteristics of major importance; i.e., porosity, microhardness, adhesion strength, and the elastic modulus of thermal spray coatings

Plasma-sprayed high entropy alloys: microstructure and properties of AlCoCrFeNi and MnCoCrFeNi

High Entropy Alloys (HEAs) represent a new class of materials that present novel phase structures and properties. Apart from bulk material consolidation methods such as casting and sintering, HEAs can also be deposited as a surface coating. In this work, thermal sprayed HEA coatings are investigated that may be used as an alternative bond coat material for a thermal barrier coating system. Nanostructured HEAs that were based on AlCoCrFeNi and MnCoCrFeNi were prepared by ball milling and then plasma sprayed. Splat studies were assessed to optimise the appropriate thermal spray parameters and spray deposits were prepared. After mechanical alloying, aluminum-based and manganese-based HEA powders revealed contrary prominences of BCC and FCC phases in their X-ray diffraction patterns. However, FCC phase was observed as the major phase present in both of the plasma-sprayed AlCoCrFeNi and MnCoCrFeNi coatings. There were also minor oxide peaks detected, which can be attributed to the high temperature processing. The measured porosity levels for AlCoCrFeNi and MnCoCrFeNi coatings were 9.5 ± 2.3 and 7.4 ± 1.3 pct, respectively. Three distinct phase contrasts, dark gray, light gray and white, were observed in the SEM images, with the white regions corresponding to retained multicomponent HEAs. The Vickers hardness (HV0.3kgf) was 4.13 ± 0.43 and 4.42 ± 0.60 GPa for AlCoCrFeNi and MnCoCrFeNi, respectively. Both type of HEAs coatings exhibited anisotropic mechanical behavior due to their lamellar, composite-type microstructure

Novel Al2CoCrFeNi high-entropy alloy coating produced using suspension high velocity air fuel (SHVAF) spraying

Metallic coatings of Al2CoCrFeNi high entropy alloy (HEA) were deposited using the suspension high velocity air fuel spray (SHVAF) process, towards exploring its viability as a bond coat in thermal barrier coatings. The relatively high Al content promoted a BCC + B2 phase-dominated coating structure, leading to enhanced mechanical properties. The oxidized microstructure exhibited a protective Al2O3 layer with characteristics comparable to conventional bond coat alloys. CC BY 4.0</p