Substrate Specimens Research Articles

Analysis of Cracking and Delamination of Sputtered Thin Films during Wear Process

Pure titanium substrate specimen was sputter-coated with SiC or CoCrNi-oxide thin film and the wear test was carried out by using a “ball-on-disk” type testing machine. The semi-circular parallel cracks appeared on the film surface with angles of 45 degree to the sliding direction of the SiC ball and the delamination of film quickly occurred after the cracking. With the aid of finite element methods, stress distribution before and after cracking in the film was obtained. The maximum tensile stress existing in the film at the back-contact edge of ball is the reason for the initiation of semi-circular parallel cracks. After the film is cracked, the tensile stress normal to interface as well as the shear stress along interface appear at crack tip. The combination of these two stresses is the main reason for the delamination after the film is cracked.

Through-thickness dependence of in-plane cracking behavior in plasma-sprayed thermal barrier coatings

In-plane macrocrack initiation of as-deposited, thermally aged, and in-service used thermal barrier coatings (TBCs) with different levels of pores and microcracks, affecting the Young’s modulus (Ec), have been studied using a newly developed protruded TBC bending technique in conjunction with a finite-element stress analysis. In-plane macrocracking along splat boundaries initiates from a curved edge of protruded TBC under the out-of-plane and shear applied and residual stresses resolved from the in-plane components. The TBC macrocracking is characterized by the critical strain energy density [(σ*e)2/Ec], which represents the material propensity, i.e., the ratio of the interfacial cohesive energy to the critical microcrack size (γf/ac), and the residual stress effect. The cracking behavior strongly depends on the type of TBC, subjected to different thermal history, and the substrate machining. Ungrooved substrate specimen tests indicate that while the as-deposited TBC has through-thickness fluctuation in (σ*e)2/Ec, the aged and used TBC exhibits a trough and peak of (σ*e)2/Ec near the middle section, respectively. In grooved or thinner substrate specimens of the TBCs where the residual stress is partially relieved, cracking further away from the interface occurs at lower and constant (σ*e)2/Ec representing γf/ac. It is interpreted that the through-thickness variations of (σ*e)2/Ec in the ungrooved substrate specimen of the various TBCs are more largely affected by the out-of-plane compressive residual stress than γf/ac.

Friction and Wear Properties of SiC-Ti and SiC-Al Films Deposited by Simultaneous RF Magnetron Sputtering

In order to improve friction and wear properties of amorphous SiC film deposited on titanium substrate specimen, Ti or Al was added in SiC film by simultaneous sputtering of SiC and Ti or Al targets using a dual-cathode RF magnetron sputtering apparatus. Wear tests were carried out on the specimens using a ball-on-disk type testing apparatus with SiC ball. The result shows that the addition of a certain amount of Ti or Al decreases wear rate of SiC film and increases the number of rotation cycles to delamination. When the Ti concentration is 2.6 at.%, the friction coefficient decreases to a minimum value of 0.04, which is about a half of SiC film without Ti and diamond-like carbon (DLC) film. This lower friction coefficient of SiC-2.6%Ti film than DLC film is obtained also for other ceramics and metal balls. The SiC-1.5%Al film reveals higher wear resistance than the SiC or SiC-Ti films with low friction coefficient of 0.1. Observation of wear scar in the film and SiC ball during the wear test implies that wear debris plays an important role in reducing the friction coefficient. Nano-indentation tests shows that the stiffness and elastic modulus of SiC-1.5%Ti and SiC-3.6%Ti is the largest among other SiC films with different Ti or Al content.

Evaluation of thermal conductivity of zirconia coating layers deposited by EB-PVD

Electron beam-physical vapor deposition (EB-PVD) is a widely used technique for depositing thermal barrier coatings (TBCs) on metal substrates for high temperature applications, such as gas turbines, in order to improve thermal efficiency [1]. Characterization of the thermal conductivity of the coating layers is therefore very important for developing superior thermal barrier coatings, but because of the irregular nature of the coated specimens it is difficult to derive the thermal conductivity of the coating layer from measurements of the thermal conductivity of the combined coating and substrate. Two steps are therefore involved in determining the thermal conductivity of a thin coating film: (i) separation of the coating film from the combined coating and substrate specimen, and (ii) measurement of the thermal conductivity of the film. With regards to the first step, it is known that coating layers deposited by EB-PVD have a porous structure so that they are easily damaged because of their poor strength [2, 3]. In other words, in practice it is not easy to physically separate the coating film from the coated substrate by machining or some other method without damaging it. Regarding the second step, even if the coating can be successfully separated from the substrate, it is not a simple matter to measure directly the thermal conductivity of the coating. The laser flash method is generally used to accurately measure the thermal diffusivity and specific heat capacity of materials, from which the thermal conductivity can be calculated. The technique was developed by Parker et al. [4], and is usually carried out assuming the specimen to be uniformly dense and opaque. However, coated layers deposited by EB-PVD have a columnar non-uniform structure, making it difficult to measure the thermal conductivity directly. The aim of the present work is therefore to derive a practical method for determining the thermal conductivity of coating layers based on theoretical calculations, and comparing the values obtained with direct experimental measurements. We have therefore adopted the response function method as a means of determining the thermal conductivity of the coating layers. It has been reported that the response function method is a powerful method to analyze one-dimensional heat diffusion across multi-layer materials [5]. We also present the experimental results from thermal conductivity measurements of coated substrates as well as coating layers detached from their substrates as a function of substrate thickness. In this work, ZrO2-4 mol% Y2O3 coatings have been applied by EB-PVD to zirconia substrates with the same composition as the coating material to minimize interface effects on thermal conductivity. Disc-type zirconia substrates were prepared by pressureless sintering at 1600◦C. The sintered substrates were machined to 10.0 mm diameter and 0.1–3 mm thickness. The substrates were first preheated at 900–1000 ◦C in a heating chamber using a graphite heating element. An electron beam evaporation process was used to deposit the film in a coating chamber under a vacuum level of 10−4 Pa using a 45 kW electron gun at a rate of 4 μm/min and substrate rotation speed of 5 rpm. The average coating thickness was about 300 μm. The density of each specimen was determined by measuring its mass on an electronic balance and its volume with a micrometer. All thermal diffusivity and specific heat capacity measurements were carried out three times for each specimen at room temperature by the laser flash method. The microstructure of the coated specimens was observed by SEM. A typical microstructure of a specimen coated on a zirconia substrate is shown in Fig. 1. The crosssectional surface of the coated specimen clearly reveals the columnar microstructure, with all columnar grains oriented in the same direction, i.e., perpendicular to the substrate. This columnar structure is very similar to those reported for metal substrates coated by EB-PVD [3]. In other words, the distinctive columnar microstructure can be obtained regardless of whether the substrate being used is metal or ceramic. The correlation between temperature rise at the rear surface of a specimen and time is shown in Fig. 2 when the front surface of the specimen is uniformly heated using a laser pulse. For bulk materials, the thermal diffusivity (α) is described by the following equation:

PEEM, AES and SXES Studies of Cu(Film)/3C-SiC

PEEM (photoemission electron microscopy), AES (Auger electron spectroscopy) and SXES (soft X-ray emission spectroscopy) studies have been carried out on Cu(thin film)/3C-SiC(substrate) contact systems. The PEEM studies have clarified both lateral diffusion and microstructure formation on the Cu/3C-SiC specimen surfaces due to heat treatment. The AES has shown accumulation of carbon atoms on top of the specimen surface at high temperature annealing. The SXES studies have elucidated a silicide, i.e., Cu3Si, formation for the heat treated Cu(film)/3C-SiC specimen. A structural model is proposed for the heat treated Cu(30 nm)/3C-SiC(substrate) specimen. [DOI: 10.1380/ejssnt.2004.230]

高圧高速フレーム溶射ＷＣ‐Ｃｏ皮膜のはく離強度に及ぼす基材脱炭層の影響

Decarburized layer was artificially formed on substrate specimens of tool steel (JIS: SKD5) by controlling the austenitizing time before quenching. The specimens were coated with WC-Co cermet by high-pressure high-velocity oxygen fuel (HP-HVOF) spraying. Tensile and edge-indent tests were carried out to evaluate the effect of decarburized layer on delamination strength of coating. During the tensile test, the coating is cracked parallelly many times with increasing load and finally delaminated along the interface between coating and decarburized layer. The tensile strength σc of coating has no remarkable change with decarburized layer thickness, while the interfacial fracture toughness Gc1/2 is lager for decarburized specimen than non-decarburized one and the Gc1/2 increases to an almost constant value with increasing layer thickness beyond 0.2mm. A Two-dimensional finite element method analysis reveals that the tensile stress and shear stress at the crack tip which are perpendicular and parallel to the interface and the tensile stress at the center of the coating are decreased by the existence of decarburized layer. The delamination energy Ed of decarburized specimens obtained by edge indent test is lower than that of non-decarburized specimen. The reason is that the decarburized layer is easy to be sheared during penetration of indenter.

Relation between Friction and Wear Properties and Delamination Behavior of Amorphous SiC Film Coated by Helicon Sputtering

SiC films were deposited on titanium substrate specimens by using a helicon-sputtering apparatus. Wear tests were carried out on the specimens using a pin-on-disk type testing machine, and the delamination behavior of the films during the tests was discussed from a viewpoint of fracture mechanics. Amorphous SiC film is formed by the sputtering, while crystalline SiC film is formed when the substrate is heated. The wear resistance of titanium is greatly enhanced by the amorphous SiC film with lower frictional coefficient and higher interfacial fracture toughness than the crystalline SiC film. The wear depth of the amorphous SiC at delamination is much deeper than that of the crystalline SiC film. In the wear test, parallel cracks are initiated perpendicular to the frictional direction before partial delamination of the film occurs, and the delamination area extends all over the wear scar with a small increase in rotation cycles. The delamination of film starts when the interfacial strain-energy release rate, G12, reaches the interfacial fracture toughness, GC12, which is decreased by the accumulation of fatigue damage during the wear cycles.

Wear behavior of nitrocarburized JIS SKD61 tool steel

The surface wear characteristics of the nitrocarburized JIS SKD61 tool steel were investigated. The ferritic substrate (FS) JIS SKD61 tool steel as received was pretreated in quenching and tempering processes to obtain the tempered martensitic microstructure. Specimens were then nitrocarburized for 1, 3, and 5 h. The chemistry, microstructure, and hardness of the nitrocarburized coatings were analyzed. Wear behavior was examined on a block-on-roller wear tester under unlubricated conditions. The results have shown that white layers were predominantly composed of the ϵ-phase iron nitrides dispersed with γ′-phase, and a large amount of nitrogen and carbon trapped in the ferrite phase was observed in the diffusion layer for FS. The nitrocarburizing treatment increased the surface roughness, hardness and the friction coefficients of the specimens. Wear contact pressures and nitrocarburized holding time also affected the friction coefficients of the nitrocarburized specimens. In this study, both the FS and the tempered martensitic substrate (TMS) specimens treated with subsequent nitrocarburizing process have shown significant improvement of the wear resistance. Wear behaviors of the specimens nitrocarburized for 1, 3, and 5 h did not vary significantly.

A method for estimation of urease activity in gastric mucosa biopsy specimens and Helicobacter pylori cell suspensions.

A method for measuring urease activity in biopsy specimens and Helicobacter pylori cultures from these specimens is proposed. The method is based on measurement (with a portable pH-meter) of the rate of pH changes in a reaction mixture consisting of buffer, substrate (urea), and biopsy specimen or bacterial cells. This method revealed that urease activity of biopsy specimens correlated with that of H. pylori suspension in the same experiment. High urease activity was found in biopsy specimens containing the greatest number of Helicobacter cells; only one of 14 specimens free of H. pylori cells showed no urease activity. Introduction of this method into clinical practice will help to evaluate the contribution of H. pylori to the pathological process.

高圧高速フレーム溶射ＷＣ‐Ｃｏ皮膜のはく離強度に及ぼす繰返し面圧の影響

Substrate specimens of mild steel (JIS: SS400) and tool steel (JIS: SKD5) were coated with WC-Co cermet by High-Pressure High-Velocity Oxygen Fuel (HP-HVOF) spraying. Repeating pressure was applied on the surface of the specimens, and tensile tests and edge-indent tests were carried out to evaluate the tensile strength and the delamination strength of the coating.The tensile strength σC of WC-Co coating increases first and then decreases with increasing surface pressure cycles N when the maximum repeating pressure σmax is 240MPa, while σC increases with increasing N for σmax=480MPa. Both the interfacial fracture toughness Gc1/2 and the delamination strength Ed for SS400 specimens decreases with increasing N. On the other hand, Ed for SKD5 specimens does not change with N in each σmax. The SEM micrographs of cross section of coating show the existence of cracks parallel to the interface for SS400 specimens and they are developed with increasing pressure cycles, while the crack extension is small for SKD5 specimens.

Preparation of Calcium Phosphate Coatings on Titanium Using the Thermal Substrate Method and Their <I>in vitro</I> Evaluation

Hydroxyapatite (Ca10(PO4)6(OH)2, HAp) and dicalcium phosphate anhydrous (Ca(H2PO4)2, DCPA) were coated onto titanium substrates using the thermal substrate method in an aqueous solution containing calcium and phosphate ions at 150°C with pH values in the range 4–8. Specimens with the HAp and DCPA layer were immersed in a simulated body fluid (SBF) to examine the dissolution and induced HAp growth behaviour of the coated layers in the SBF . No precipitation occurred on the coated DCPA layer after 7 days immersion in the SBF, and also the DCPA did not dissolve in the SBF . On the other hand, a new precipitate, which was identified as HAp, nucleated after 1 day and grew in a spherical manner on soaking the HAp substrate specimens in the SBF . The influence of small quantities of DCPA in the HAp layer on the coating’s bioactive property in the SBF was negligible as the DCPA was not on the surface of the coated layer.

The influence of additions of Nb and Cr on the aluminizing behavior of TiAl alloy

The intermetallic compound TiAl is expected to be one of the candidates for high-temperature systems such as aircraft engines because it has a high specific strength at elevated temperatures.[1,2] However, there are two major drawbacks with the intermetallic compound TiAl as an engineering material: the first is its poor room-temperature ductility and the other is its poor oxidation resistance above 800 8C. The former has been generally overcome by modification of the microstructure using ternary element addition such as Nb, Si, or W. However, improvement in the oxidation resistance by addition of these alloying elements is not enough for practical application. Furthermore, the alloying elements may have a deleterious effect on the mechanical properties of the material. Therefore, surface treatments are thought to be more attractive. Fig. 4—Typical Cr31 photoluminescence spectra acquired from thermal Several studies on the surface treatments have been perbarrier coated (MCrAlY bond coat and EB-PVD YSZ ceramic coat) turbine formed to improve the oxidation resistance of TiAl. The blades before and after the engine test. surface treatments contain MCrAlY overlay coating,[3,4] aluminizing,[5,6] silicide/ceramics,[7,8] and preoxiding at low oxygen pressure.[9] However, none of them seems suffiapplication of Cr31 photoluminescence piezospectroscopy ciently successful. is currently being investigated as a nondestructive inspection Aluminide coatings are widely used for high-temperature technique for TBCs. materials because of their practical advantages over other coatings. The pack aluminizing of TiAl basically produces TiAl3 coating. However, TiAl3 is reported to be more brittle than TiAl. During cyclic oxidation under thermal stress, the The authors thank Drs. G. Wagner and D.B. Allen, SieTiAl3 layer is easily spalled off due to its brittle nature. The mens-Westinghouse Power Corporation, as well as the additions of Nb and Cr to TiAl and TiAl3 alloys have been technical staff at Renishaw, Inc., for helpful discussions shown to be beneficial for mechanical properties as well as and experimental collaborations. Financial support, profor oxidation resistance.[10] However, the effect of the alloyvided by the Department of Energy under the Advanced ing element contained in the TiAl substrate on the aluminizGas Turbine Systems Research Program (Grant No. ing behavior of TiAl alloy has not been extensively AGTSR 99-01-SR073), is gratefully acknowledged. studied yet. In this investigation, pack aluminizing is studied with the TiAl alloy. Emphasis is placed on the effect of alloying REFERENCES elements on the aluminizing behavior of TiAl alloy. To inves1. S.M. Meier, D.M. Nissley, K.D. Sheffler, and S. Bose: J. Eng. Gas tigate the effect of alloying elements, two different samples Turbines Power, 1992, vol. 114, pp. 259-305. of the non-alloyed TiAl and TiAl alloyed with Nb and Cr 2. J.T. DeMasi-Marcin, K.D. Sheffler, and S. Bose: J. Eng. Gas Turbines are used. Power, 1990, vol. 112, pp. 521-26. The specimens were prepared by melting high-purity alu3. R.A. Miller: Surf. Coating Technol., 1987, vol. 30, pp. 1-11. 4. R.D. Sisson, Jr., E.Y. Lee, and Y.H. Sohn: Proc. 2nd Pacific Rim Int. minum, titanium, niobium, and chromium grains in a vacuum Conf. on Advanced Materials and Processing, K.S. Shin, J.K. Yoon, arc furnace. The ingots were turned and remelted five times. and S.J. Kim, eds., The Korean Institute of Metals and Materials, After homogenization treatment of the ingots at 1100 8C for Kyung-Joo, Korea, 1995 pp. 1203-10. 24 hours, substrate specimens were prepared by cutting the 5. Q. Ma and D.R. Clarke: J. Am. Ceram. Soc., 1994, vol. 77, pp. 298-302. 6. D.M. Lipkin and D.R. Clarke: Oxid. Met., 1996, vol. 45, pp. 267-80. ingots into coupons of approximate dimensions 12 3 10 7. J.R. Christensen, D.M. Lipkin, D.R. Clarke, and K. Murphy: Appl. 3 2 mm, grinding their surfaces to a 600-grit finish, and Phys. Lett., 1996, vol. 69, pp. 3754-56. ultrasonically cleaning with acetone. The compositions of 8. M. Gell, K. Vaidyanathan, B. Barber, J. Cheng, and E. Jordan: Metall. Mater. Trans. A, 1999, vol. 30A, pp. 427-35. 9. M. Gell, E. Jordan, K. Vaidyanathan, K. McCarron, B. Barber, Y.H. Sohn, and V.K. Tolpygo: Surf. Coating Technol., 1999, vols. 120–121, CHUNGEN ZHOU, Associate Professor, and HUIBIN XU, Professor, are with the Department of Materials Science and Engineering, Beijing pp. 53-60. 10. M.J. Lance, J.A. Haynes, W.R. Cannon, and M.K. Ferber: in Ceramics University of Aeronautics and Astronautics, Beijing 100083, People’s Republic of China. KYOO YOUNG KIM, Professor, is with the Department Transactions: Non-Destructive Evaluation of Ceramics, C.H. Schilling and J.N. Gray, eds., American Ceramics Society, Westerville, OH, of Materials Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea. 1998, vol. 89, pp. 229-36. 11. D.R. Clarke and W. Pompe: Acta. Mater., 1999, vol. 47, pp. 1749-56. Manuscript submitted September 7, 1999.

Tribological properties of pulsed ion nitrided AISI 4140 steel

The present study was conducted in order to investigate the tribological behaviour of plasma nitrided AISI 4140 steel by using a dc pulsed glow discharge technique. Microstructure and structural characterisations were performed by using scanning electron microscopy and X-ray diffraction techniques. A compound layer with a thickness between 6 and 8 μm containing ɛ (Fe3 N) and to a lesser extent γ′ (Fe4 N) was identified. Wear tests were performed with a pin on disc configuration. The tests were carried out at room temperature in air using two different pin materials (AISI 1086 and WC–6Co). A sliding speed of 0·1 m s-1, 5 N load, and 700 m maximum sliding distance were used on both substrate and nitrided specimens. The friction coefficients found for the tribological pairs under study, tungsten carbide pin and plasma nitrided disc, AISI 1086 pin and plasma nitrided disc, and AISI 1086 pin and AISI 4140 disc, were 0·34–0·6, 0·49–0·6, and 0·475–0·57 respectively. Wear volumes of 0·0453, 0·0511, and 0·00077 mm3 have been determined for the tribological pairs mentioned above. The surface top ography of the wear scars on both specimens and pins have been determined using scanning electron micros copy and three-dimensional stylus profilometry.

Role of chromium carbide in tungsten carbide based hard composites

The surface wear characteristics of the nitrocarburized JIS SKD61 tool steel were investigated. The ferritic substrate (FS) JIS SKD61 tool steel as received was pretreated in quenching and tempering processes to obtain the tempered martensitic microstructure. Specimens were then nitrocarburized for 1, 3, and 5 h. The chemistry, microstructure, and hardness of the nitrocarburized coatings were analyzed. Wear behavior was examined on a block-on-roller wear tester under unlubricated conditions. The results have shown that white layers were predominantly composed of the ϵ-phase iron nitrides dispersed with γ′-phase, and a large amount of nitrogen and carbon trapped in the ferrite phase was observed in the diffusion layer for FS. The nitrocarburizing treatment increased the surface roughness, hardness and the friction coefficients of the specimens. Wear contact pressures and nitrocarburized holding time also affected the friction coefficients of the nitrocarburized specimens. In this study, both the FS and the tempered martensitic substrate (TMS) specimens treated with subsequent nitrocarburizing process have shown significant improvement of the wear resistance. Wear behaviors of the specimens nitrocarburized for 1, 3, and 5 h did not vary significantly.

Mechanical stability of DLC films on metallic substrates Part II — Interfacial toughness, debonding and blistering

DLC coatings have been deposited onto four metallic substrates, and also onto silicon, with the interfaces being prepared in various ways. Films with a range of thicknesses have been produced and specimens have been subjected to changes in temperature. From a knowledge of the residual stress levels in these films as a function of temperature (see Part I), the strain energy release rate for interfacial debonding has been monitored during deposition and subsequent temperature changes. The value of the strain energy release rate at a point when interfacial debonding occurred has been taken as the interfacial toughness (fracture energy). It is concluded from these observations that the interfacial toughness is low for DLC films deposited onto mechanically-polished surfaces of titanium, mild steel or stainless steel, but quite high on aluminium. Pre-cleaning by bombardment with energetic argon ions raises the interfacial toughness for the steels and for aluminium, but not for titanium. These observations have been related to the nature of the oxide films on these substrates. A thin sputtered interlayer of aluminium was found to raise the interfacial toughness significantly for the steels and for titanium. However, experiments with silicon substrate specimens showed that the good interfacial adhesion obtained with Al interlayers is severely impaired by heating in vacuum to temperatures of the order of 500°C. It has been established that this impairment of adhesion is a consequence of the release of Ar entrapped in the Al during sputter deposition, which promotes the formation of blisters and detachment of the film.

Characteristics of residual stress generated by MoSi2 plasma thermal spraying

Summary This paper describes an investigation of plasma thermal spraying of MoSi2 offering a high melting point together with excellent high-temperature oxidation resistance. MoSi2 powder was deposited by low-pressure plasma thermal spraying on the surfaces of blasted SS400 substrate specimens precoated with 50%Ni-50%Cr. The residual stress generated in the specimens was then determined. Compressive stress is generated in the MoSi2 sprayed deposit. Tensile stress is generated in the 50%Ni-50%Cr sprayed deposit except at the SS400 substrate surface. A series of experiments was conducted to clarify the validity of the residual stress distribution obtained. These results suggest the following: 1. When only blasting treatment is performed, compressive stress is generated from the upper surface to 0.3 mm inside. Minor tensile stress is also generated inside. 2. When 50%Ni-50%Cr is deposited after blasting treatment, the residual stress is tensile in the 50%Ni-50%Cr layer and compressive on the substrate upper surface. Tensile stress is found inside. This is due to the fact that the substrate restrains the contraction of the sprayed deposit with a high thermal shrinkage. The validity of this result is also supported by the fact that the compressive stress near the substrate upper surface is greater than when only blasting treatment is performed. 3. When MoSi2 is deposited after blasting treatment, compressive stress is generated in the MoSi2 layer. This is due to the fact that, despite the MoSi2 layer having a higher temperature than the substrate, the linear thermal expansion coefficient of MoSi2 is much smaller than that of the substrate, so that the MoSi2 layer restrains the contraction of the substrate. 4. During estimation of the residual stress in materials deposited by plasma thermal spraying, it is important to ensure that the modulus of elasticity of the sprayed deposit is measured separately for each set of thermal spraying conditions.

Determination of processing-induced stresses and properties of layered and graded coatings: Experimental method and results for plasma-sprayed NiAl 2O 3

An experimental method is proposed which enables the determination of processing-induced intrinsic stresses, elastic modulus, and coefficients of thermal expansion of surface coatings of homogeneous and graded compositions. In this method, a number of identical substrate specimens are coated simultaneously with surface layers of fixed or graded compositions, and specimens with different layer thicknesses are periodically removed from the deposition chamber. It is shown that the following results can be obtained from a knowledge of the strain or curvature and thermal history of the coated specimens, in conjunction with simple four-point bend tests and thermal loading/cycling at different temperatures: (i) the magnitude of the processing-induced intrinsic stresses through the thickness of the coating, (ii) the in-plane Young's modulus, E, as a function of the coating thickness, (iii) the coefficient of thermal expansion, α, as a function of the coating thickness, (iv) the variation of E and α as a function of temperature at any thickness location within the coating, and (v) the separation of internal stresses arising from thermal expansion mismatch between different constituent phases or layers from those arising from the deposition process (so-called “intrinsic” or “quench” stresses). The thermomechanical analyses underlying this method are discussed in detail, and its significance and limitations are addressed. The proposed method is used to determine the evolution of processing-induced stresses during the successive build-up of plasma-sprayed NiAl 2O 3 coatings of homogeneous and graded compositions.

Fatigue crack growth properties of ceramic coated 1Cr-1Mo-0.25V steel

Fatigue tests were performed to investigate the effect of ceramic coatings as in TiN and TiCN on fatigue crack growth properties of ceramic coated 1Cr-1Mo-0.25V steel with different coating thickness in laboratory air conditions. The experimental results are described with respect to a Paris equation, da/dN=C(.DELTA.K) , where the crack growth rate of coated specimens provided as similar growth rate as that of the uncoated specimen regardless of coating thickness. Furthermore, it was observed that the type of coating layer had virtually no effect on crack growth rate in the full region of stress intensity factor range. And it was also appeared that the final crack length of TiCN coated specimens was short compared to that of TiN coated, and the substrate specimens, in which it was inferred due to lowering the toughness of coated material from high hardness of TiCN coating layer itself.

Epitaxy and morphology of deposited α-Fe films on high-index planes of Cu substrates

Abstract Thin α-Fe films were deposited onto the high-index planes {212}, {103}, {113}, {203} and {213} of Cu substrates. Multiple epitaxial relationships were found to exist in each substrate specimen. However, the α-Fe planes of the deposited layer were always related to the Cu substrate planes by the Bain correspondence. Particular growth directions of the α-Fe films parallel to 〈110〉 on the Cu substrates were observed for the {212} and {113} substrate specimens. Simple geometrical criteria proposed previously to explain the epitaxial relationships between low-index planes are also highly applicable to the prediction of the epitaxies between high-index planes. Three-dimensional orientation relationships, rather than two-dimensional orientation relationships at the interface, must be considered in order to understand the crystallography and morphology of the α-Fe films.

Influence of the surface structure of titanium materials on the adhesion of fibroblasts

Of utmost importance for the successful use of an implant is a good adhesion of the surrounding tissue to the biomaterial. In addition to the surface composition of the implant, the surface topography also influences the properties of the adherent cells. The aim of this investigation was thus to study the influence of the surface structure of the substrate on the formation of focal contacts and on the orientation of cultivated gingival fibroblasts by means of fluorescence microscopy. A further goal was to determine the effect of the material composition on the cell shape, on the assumption that in each case a lengthening of the cells can be expected to provide a more favourable adhesion behaviour than a spherical cell shape. In order to describe the shape of the cell, a shape factor was defined which was calculated from the area covered by the cells and from their circumference. To determine the influence of the surface structure, substrate platelets of cp-titanium, TiAl6V4 and TiTa30 were ground. Onto these specimens human gingival fibroblasts of the 5th to 7th passages were cultivated. After a culture time of two days the cells were fixed and stained. The number of orientated cells was determined as a function of the surface roughness of the substrate. The number of orientated cells was shown to increase — independent of the material — with increasing roughness of the ground substrate. On a polished surface the number of orientated cells was 11% (average peak-to-valley height 0.04 μm); at a peak-to-valley height of 1.36 μm the number of orientated cells increased to 72%. It could be observed that the orientated cells had a higher density of focal contacts where they were in contact with the edges of the grinding grooves. In order to determine the effect of the surface composition, gingival fibroblasts were cultured for 14d on polished substrate specimens of cp-titanium, TiAl6V4 and TiTa30 and examined for differences in shape. The cells grown on cp-titanium and on TiTa30 had shape factors of 1.76 and 1.58 respectively, whereas those grown on TiAl6V4 had a shape factor of 0.93. The unfavourable spherical shape of the fibroblasts (resulting in a poor adhesion) grown on TiAl6V4 after a culture period of 14 d may be the result of a locally increased vanadium concentration in the substrate, with an accompanying increase in the release of toxic vanadium ions.