Significance of quenching stress in the cohesion and adhesion of thermally sprayed coatings

In thermal spraying, molten particles strike a solid surface, where they are flattened and quenched within a very short time. Considerable in-plane tensile stress on the order of 100 MPa can develop within each splat during quenching after solidification because thermal contraction of the particle is constrained by the underlying solid. Ni-20Cr alloy and alumina powders have been plasma sprayed in air onto steel substrates that were maintained at about 473 K. The influence of spraying conditions such as spray distance on the magnitude of the quenching stress have been studied by measuring the curvature of the substrate during spraying. Mechanical properties such as Young’s modulus and bend strength of the deposited coatings have also been measured. A strong correlation was found between the quenching stress and the strength of Ni-20Cr coatings, which suggests that the strength of interlamellar bonding limits the quenching stress at such temperature. Presented at ITSC ’92, June, 1992, Orlando, Florida.     

Dependency of fracture toughness of plasma sprayed Al<Subscript>2</Subscript>O<Subscript>3</Subscript> coatings on lamellar structure

The fracture toughness of plasma-sprayed Al₂O₃ coatings in terms of critical strain energy release rate G _Ic was investigated using a tapered double cantilever beam (TDCB) approach. This approach makes the fracture toughness be measured only using the critical fracture load disregarding crack length during test. The Al₂O₃ coatings were deposited under different spray distances and plasma powers to clarify the effect of spray parameters on the G _Ic of the coatings. The fracture surfaces were examined using scanning electron microscope. On the basis of an idealized layer microstructure model for thermal sprayed coatings, the theoretical relationship between the cohesive fracture toughness and microstructure is proposed. The correlation between the calculated fracture toughness and observed value is examined. It was found that the fracture toughness of plasma sprayed Al₂O₃ coatings is not significantly influenced by spray distance up to 110 mm, and further increase in spray distance to 130 mm resulted in large decrease in the fracture toughness of the coatings. The G _Ic value predicted based on the proposed model using lamellar interface mean bonding ratio and the effective surface energy of bulk ceramics agreed well with the observed G _Ic data. Such agreement evidently shows that the fracture toughness of thermally sprayed ceramic coatings at the direction along coating surface is determined by lamellar interface bonding.     

Mechanical and thermoelectric properties of Zn4Sb3 and Zn4Sb3+Zn directly synthesized using elemental powders

Direct synthesis using elemental powders has been used to produce single-phase polycrystalline ε-Zn₄Sb₃ specimens as well as composite specimens having ε-Zn₄Sb₃ (majority phase) and Zn (minority phase). The effect of the Zn phase on the elastic, thermoelectric and mechanical properties was investigated in this alloy system. Thermoelectric properties of single-phase Zn₄Sb₃ at an ambient temperature are comparable to the published data for the sample prepared by a hot-pressing of ingot-melted alloy powders. Transport properties at room temperature were also evaluated. In addition, Young’s modulus and the bulk modulus of polycrystalline Zn₄Sb₃ were measured using a resonant-ultrasonic technique. The fracture toughness in this alloy system was determined by measuring the length of cracks that formed at the corners of pyramidal indentations used for hardness tests. It is shown that the addition of Zn increases the fracture toughness, but this is achieved at the cost of reducing the thermoelectric figure of merit.     

Mechanical and tribological properties of plasma-sprayed Cr3C2-NiCr, WC-Co, and Cr2O3 coatings

Mechanical properties such as Young’s moduli and fracture toughness of plasma-sprayed Cr₃C₂-NiCr, WC-Co and Cr₂O₃ coatings were measured. The tribological properties of the three kinds of coatings were investigated with a block-on-ring self-mated arrangement under water-lubricated sliding. Furthermore, the influences of the mechanical properties on the tribological properties of the coatings were also examined. It was found that the Young’s moduli, bend strengths and fracture toughness of the coatings were lower than the corresponding bulk materials, which may be attributed to the existence of pores and microcracks in the coatings. Among the three kinds of coatings, the magnitude of wear coefficients, in decreasing order, is Cr₃C₂-NiCr, WC-Co and Cr₂O₃, and the wear coefficient of Cr₂O₃ coating was less than 1 × 10⁻⁶mm³N⁻¹m⁻¹. The wear mechanisms of the coatings were explained in terms of microcracking and fracturing, and water deteriorated wear performance of the coatings. The higher the fracture toughness and the lower the porosity and length of microcracking of the coating, the more the wear-resistance of the coating.     

Technology of thermally sprayed anilox rolls: State of art,problems, and perspectives

This paper deals with the surfacing technology of ceramic anilox rolls. The rolls are used in the printing industry to transport the precisely determined quantity of ink in the flexographic printing machines. The technology of roll surfacing is discussed by taking the following aspects into account: preparation of the powder to spray the ceramic coating; thermal spraying of the duplex (bond coating and ceramic top coating); postspray finishing by grinding and polishing; and laser engraving. The powder used as the top coating of the aniloxes is chromium oxide. This powder might be prepared by such techniques as agglomeration, fusing, and crushing, etc. The preparation technique influences coating properties, such as microstructure (tested with SEM, OM, XRD, and XPS), open porosity, microhardness, and modulus of elasticity. Comparison of these properties enables optimum powder preparation techniques to be found. APS technique is used to coat the anilox rolls. Optimization of the plasma spraying parameters is discussed. Aniloxes are submitted to the grinding and polishing of the ceramic coating before laser engraving occurs. The final roughness of the finished coating is discussed in view of an optimum absorption of the laser light energy at engraving. Possible ways of reducing the spraying time are discussed, and future research toward improving the anilox roll quality is proposed.     

三元共晶成分Al2O3/YAG/ZrO2粉体及陶瓷的制备与组织性能研究

本文采用醇水共沉淀法制备了三元共晶成分Al₂O₃/YAG/ZrO₂粉体,在600-1350_oC温度范围煅烧后研究其物相转变过程。经1300_oC煅烧后Al₂O₃/YAG/ZrO₂共晶成分粉体的物相由α-Al₂O₃、c-ZrO₂和YAG构成,且具有α-Al₂O₃相包裹c-ZrO₂相的特殊结构。将煅烧粉体在1550_oC下热压烧结,制备具有内晶型结构的共晶成分Al₂O₃/YAG/ZrO₂复相陶瓷,其致密度、室温抗弯强度、断裂韧性和高温(1000_oC)抗弯强度分别为98.8%、420 MPa、3.69 MPa.m_1/2和464 MPa,并对复相陶瓷组织结构的形成机理进行了探讨。     

Cu+B复合钎料配比对Al2O3/TC4合金钎焊接头界面组织的影响

分别以不同配比的铜和硼混合粉末作为钎料,在钎焊温度930℃,保温时间10min条件下钎焊连接Al2O3与TC4合金,并结合SEM与EDS观察对钎焊接头的连接状况及组织结构进行分析．结果表明,A12O3／TC4合金钎焊接头的界面组织为Al2O3,／Ti3（Cu,A1）3O／Ti2Cu＋Ti2（Cu,Al）／Ti＋Ti2（Cu,Al）＋Ti（Cu,Al）＋AlCu2Ti＋Ti2Cu／Ti2Cu＋AICu：Ti＋TiB／Ti＋Ti2Cu／TC4合金．钎焊过程中,复合钎料中的铜与TC4合金中的钛发生互扩散,在连接层与TC4合金界面形成不同成分的Cu-Ti化合物．硼与液相中的钛反应,在接头中原位生成TiB晶须,且主要分布在Ti2Cu和AlCu2Ti上．随TiB生成量的增加,Ti2（Cu,Al）生成量增加,并逐渐变得连续,同时向Al2O3侧移动．     

Structure and mechanical properties of plasma sprayed nanostructured alumina and FeCuAl-alumina cermet coatings

Nanostructured alumina (Al₂O₃) and nanostructured cermet coatings containing alumina dispersed in a FeCu or FeCuAl matrix, were deposited by atmospheric plasma spraying (APS) from nanostructured powders. These coatings were characterized by SEM, EDAX, TEM, XRD and nanoindentation. Friction and wear behaviour were investigated by sliding and abrasion tests. TEM and XRD revealed that a nanostructuring was retained in the APS deposited coatings.The nanostructured ceramic and cermet coatings were compared in terms of coefficient of friction and wear resistance. Nanostructured cermet coatings appeared to offer a better wear resistance under sliding and abrasion tests than nanostructured Al₂O₃ coatings. The role of Fe, Cu, and Al additions to the Al₂O₃ coatings on friction and wear behaviour, was investigated.In the case of FeCu- and FeCuAl-based cermet coatings containing alumina, though the starting material consist of only two compounds, the coatings contain up to four different phases after plasma spraying. The mechanical properties of these different phases namely crack sensitivity and elasto-plastic deformation was determined by nanoindentation. The failure mechanisms were investigated and an attempt was made to establish a ‘structure-property’ relationship. It was shown that an appropriate balance between hard and soft phases results in optimum tribological properties of the nanostructured cermet coatings.     

Effect of Feedstock Size and its Distribution on the Properties of Detonation Sprayed Coatings

The detonation spraying is one of the most promising thermal spray variants for depositing wear and corrosion resistant coatings. The ceramic (Al₂O₃), metallic (Ni-20 wt%Cr) , and cermets (WC-12 wt%Co) powders that are commercially available were separated into coarser and finer size ranges with relatively narrow size distribution by employing centrifugal air classifier. The coatings were deposited using detonation spray technique. The effect of particle size and its distribution on the coating properties were examined. The surface roughness and porosity increased with increasing powder particle size for all the coatings consistently. The feedstock size was also found to influence the phase composition of Al₂O₃ and WC-Co coatings; however does not influence the phase composition of Ni-Cr coatings. The associated phase change and %porosity of the coatings imparted considerable variation in the coating hardness, fracture toughness, and wear properties. The fine and narrow size range WC-Co coating exhibited superior wear resistance. The coarse and narrow size distribution Al₂O₃ coating exhibited better performance under abrasion and sliding wear modes however under erosion wear mode the as-received Al₂O₃ coating exhibited better performance. In the case of metallic (Ni-Cr) coatings, the coatings deposited using coarser powder exhibited marginally lower-wear rate under abrasion and sliding wear modes. However, under erosion wear mode, the coating deposited using finer particle size exhibited considerably lower-wear rate.