Erja Turunen

Comparison between plasma- and HVOF-sprayed ceramic coatings. Part I: microstructure and mechanical properties

Few papers deal with High Velocity Oxygen-Fuel (HVOF) sprayed ceramics. This two-part study thoroughly compares HVOF sprayed Al2O3, nanostructured Al2O3, Cr2O3 to Atmospheric Plasma Sprayed (APS) ones. The first part discusses microstructure and micromechanics. HVOF-sprayed ceramics achieve superior cohesion (lower porosity and lower average pore area). Size effects in Vickers microindentation tests are different for HVOF and APS ceramics. At 1 N load, hardness is higher for HVOF coatings (no cracking). Under 5 N and 10 N loads, hardness decreases gradually for HVOF coatings; APS coatings are largely cracked at 5 N. HVOF-sprayed ceramics are tougher than APS ones and have higher elastic modulus.

Comparison between plasma- and HVOF-sprayed ceramic coatings. Part II: tribological behaviour

This is the second part of comparative study between High Velocity Oxygen-Fuel (HVOF) flame-sprayed and Atmospheric Plasma-Sprayed (APS) ceramics. Dry particles abrasion test and dry sliding wear test at room temperature, 400?C and 700?C are performed. In dry sliding against SiC, at room temperature stable tribofilms are formed and mild wear (10-6 mm?/(Nm)) occurs for all coatings. When temperature and normal load increase, making brittle cracking a significant wear mechanism, HVOF coatings become superior to APS ones, thanks to higher toughness. In dry particles abrasion, brittle fracture prevails; therefore, the tougher HVOF coatings outperform APS ones.

Sealing of Thermally Sprayed Coatings

Abstract The use of coatings in fluidised bed boilers is increasing owing to the harsh conditions that modern boilers meet. High temperatures, combined with inhomogeneous, high chlorine and alkali containing fuels such as biomass, cause severe material wastage to metallic parts in boilers. Thermally sprayed coatings have been reported to provide protection to boiler tubes. However, thermally sprayed coatings have encountered serious problems, such as corrosion on the substrate material owing to voids and oxides in lamella boundaries. Sealing of the coatings can solve these problems. This paper investigates the sealing of coatings. Thermally sprayed metallic coatings were sealed with different commercial sealing agents and laser treatment. The coatings were tested after sealing by simulating fluidised bed boiler superheater conditions. Specimens were examined by optical microscopy and SEM and analysed by energy dispersive X-ray analysis. Some of the tested sealants protected the coatings adequately in a short term alkali chloride– alkali sulphate exposure test. The best sealant contained aluminium oxide and aluminium phosphate. Laser treated coatings had good corrosion resistance in a short term test.

Influence of Cr alloying on the microstructure of thermally sprayed quasicrystalline Al-Cu-Fe coatings

Abstract The present work reports the structural development of Al–Cu–Fe and Al–Cu–Fe–Cr coatings deposited by the high velocity oxy-fuel thermal spraying process and the influence of Cr alloying on the phase selection of Al–Cu–Fe coatings at various deposition temperatures. The porosity levels of the Al–Cu–Fe and Al–Cu–Fe–Cr coatings of the study are demonstrated to be lower than those reported for corresponding plasma-sprayed coatings. The results show that high velocity oxy-fuel spraying technique produces Al–Cu–Fe coatings that are phase structurally similar to plasma-sprayed Al–Cu–Fe coatings reported in literature. Al–Cu–Fe coatings are composed of a crystalline β-AlFe phase and a quasicrystalline i-Al65Cu20Fe15 phase as well as an oxidised form of either or both of these phases. Addition of Cr to Al–Cu–Fe alloys introduces coatings that are made up of the crystalline θ-Al2Cu phase and two quasicrystalline phases, the i1-Al80Cr13.5Fe6.5 and i2-Al13Cr3Cu4 phases. The formation of these icosahedral phases in Al–Cu–Fe–Cr alloys has not been reported before, although the occurrence of quasicrystal approximants with compositions close to those of the i1-Al80Cr13.5Fe6.5 and i2-Al13Cr3Cu4 phases has been demonstrated. On the basis of our results we propose that the icosahedral phase structure is greatly stabilised by the Cr addition to Al–Cu–Fe alloys.

Microstructural characterisation of thermally sprayed quasicrystalline Al–Co–Fe–Cr coatings

Abstract A microstructural characterisation was carried out for Al–Co–Fe–Cr feed powder and the coatings sprayed with a high velocity oxy-fuel method using different operation conditions. The aims of the study were to explore the structural development of thick Al–Co–Fe–Cr coatings and the influence of the spraying parameters on the microstructure of produced Al–Co–Fe–Cr coatings. X-ray diffractometry, scanning electron microscopy and analytical transmission electron microscopy were the techniques used in the phase identification and in the microstructural exploration of the study. The results show that Al–Co–Fe–Cr feed powder and the coatings sprayed with low and high operation temperature are composed of a dodecagonal quasicrystalline phase. The composition of this new dodecagonal phase approximately corresponds to that of the feed powder, being A1 70.6 Co 12.5 Fe 9.4 Cr 7.5 . The dodecagonal phase does not decompose during the spraying process. Instead, it orientates to form a lamellar coating structure. When a lower spraying temperature is used, the incomplete melting of powder particles introduces a partly orientated coating structure. Due to this incomplete melting of powder particles, porosity is also involved in these coatings. Higher spraying temperature, in turn, promotes oxidation, leading to the incorporation of an oxygen-containing film on the splat boundaries. While the feed powder and the coating deposited with a lower spraying temperature are one-phase quasicrystalline structures, the coating sprayed with a higher operation temperature is comprised of a dodecagonal phase and an oxygen-containing phase. This oxygen-containing phase is not pure aluminium oxide but contains all the elements present in the alloy.

Development of Nanostructured Al2O3-Ni HVOF Coatings

HVOF thermal spraying has been developed to deposit dense Al2O3-coatings for improved protective properties. As compared to generally used plasma sprayed coatings HVOF coatings can be prepared much denser and thus are better suited for applications where protective properties of the coating are needed. In this paper we describe the development of HVOF spraying technologies for nanocrystalline Al2O3- and Al2O3-Ni-coatings. The microstructure and the mechanical properties of these novel coatings are reported and compared to a conventionally processed Al2O3-coating.

Improved Mechanical Properties by Nanoreinforced Ceramic Composite HVOF Coatings

Special nanoreinforced ceramic composite coatings were produced. Dense nanostructured alumina and chromia coatings alloyed with various nanofraction elements such as Ni, NiO, ZrO2 and SiC were manufactured by HV2000 HVOF spraying. As a result coatings with nearly 100% improvements relative fracture toughness, were produced.

Materials for Electronics by Thermal Spraying

In this paper, dielectric and conductive properties of thermally sprayed Al2O3- and Cu-based coatings on steel and alumina substrates were studied. Alumina powders with nanoand micro-sized additions of Ni, NiO, TiO2, silica, and commercial glass were used in High Velocity Oxygen Fuel (HVOF) deposition. The conventional commercial copper powder and three Ag, WC and H2 -modified powders were used in Direct Write Thermal Spray (DWTS) deposition. Mixed phases of α-Al2O3 and γ-Al2O3 were found to be present in the as-deposited coatings. Sprayed alumina-based composites exhibited dielectric permittivity of 5.3-13.9 and losses of 0.002-0.178 at 1 MHz and 1 GHz while the additions tend to increase the values. Sprayed compositions with glass-type additions were found to retain α-Al2O3 crystalline phase after the deposition. Cu depositions, especially modified ones, realised by Direct Write Thermal Spray (DWTS) showed conductivity values as high as 4256 % of IACS values. The results demonstrate that ceramic and conductive coatings fabricated by thermal spray techniques show feasible properties for electrical applications, such as low-frequency components and insulation layers to be utilised in embedded 3D circuitry, in a way that is not possible through traditional manufacturing methods.

Process Optimization for Nanostructured HVOF -Sprayed Al2O3-Based Ceramic Coatings

Special mechanical properties have widely been demonstrated with bulk nanocrystalline materials. An increasing effort has been made to transfer such improvements also into thermal sprayed ceramic coatings. This paper focuses on such efforts in alumina-based ceramic coatings. The optimization of process conditions and effect of different process parameters on the mechanical performance of high velocity oxy-fuel (HVOF) sprayed ceramic coatings is discussed.

Mechanical properties of nanocrystalline HVOF coatings

Thermal spraying (TS) is increasingly used to deposit thick wear resistant coatings on machine and equipment parts. Therefore, it is of uttermost importance to know their mechanical properties in order to tailor and optimize the coatings for various applications. Typically properties of the thermal sprayed coatings are inferior as compared to corresponding bulk materials for various reasons. Furthermore, measurement of mechanical properties of coatings is much more tedious and difficult than deriving bulk properties. In this paper, mechanical properties of alumina based nanocrystalline coatings and the measurement techniques developed are reported. A model relating the microstructure of the sprayed coatings and their properties is introduced and discussed.