Stress relaxation in the interface between creep particle and elastic matrix-strengthening mechanism

The effect of a power law creep particle on interface behavior between the particle and elastic matrix is investigated by stress analysis. Using the results obtained through the stress analysis, the forces due to interaction of an applied stress and stress concentration with an edge dislocation are determined. The direct interaction between the edge dislocation and the creeping particle is studied under fully relaxed stress conditions. Through the investigation the following results are derived. Stress relaxation in the interface can be caused by power law creep along or by diffusion, or a combination of both mechanisms. The degree of stress relaxation caused by diffusion can be defined in terms of the relaxation time for both boundary diffusion and volume diffusion. The amount of stress relaxation caused by the power law creep particle is characterized by the quantity ₂ which is a function of Γ₀ = 2(1/√3)^{1 + m} × (σ_∞/2μ)^m(σ₀/σ_∞t^m), where m is strain rate hardening exponent, σ_∞ is applied stress, μ is the shear modulus, σ₀ is the material constant of the power law creep particle, and t is elapsed time. The value ₂ = 1.0 corresponds to the fully relaxed condition and ₂ = −0.6 corresponds to the initial state. The time to reach a fully relaxed condition is very sensitive to the strain rate exponent, with the smaller m values leading to longer times. The stress state of complete relaxation in the elastic matrix is equivalent to the solution of a void in an elastic matrix superposed on the solution of positive surface traction on the void. This result is identical to that obtained by Srolovitz et al. [Acta. Metall.32, 1979 (1984)]. When the stress is completely relaxed in the particle, all stress components (σ_r, σ_θand σ_rθ) are relaxed, while in the matrix relaxations are observed only for σ_rand σ_θ. The critical resolved shear stress and critical stress to climb the dislocation in the neighborhood of the particle exceed the Orowan stress. Also, the particle attracts the dislocation. Therefore the strengthening of a power law creep particle in an elastic matrix is caused by the Orowan mechanism and by attraction of the dislocation.     

Cracking in coating-substrate composites with multi-layered and FGM coatings

This investigation evaluates, by finite element method, the stress intensity factors (SIF) of cracked multi-layered and functionally graded material (FGM) coatings of a coating-substrate composite, due to the action of uniform normal stress on the crack surfaces. The substrate is assumed to be homogeneous material, while the coating consists of multi-layered media or sigmoid FGMs. The sigmoid FGM is a kind of FGM in which the material properties of the coating are governed by two power-law functions of volume fractions such that the functions of the material property represent sigmoid distributions in the thickness direction, simply called S-FGM in this paper. For the multi-layered coatings, one, two, and four-layered homogeneous coatings with stepwise changing volume fractions are considered. The primary problem addressed herein is the appearance of a crack in the coating surface and its expansion into the substrate along the direction perpendicular to the interface between the coating and the substrate. The results show that if the coating is stiffer than the substrate, a crack in a one-layered coating is much more susceptible to propagation into the substrate than a crack in the two- or four-layered coating. But crack growth can be effectively averted by using an S-FGM coating. However, if the coating is softer than the substrate, the S-FGM coating behaves like a bridge to connect the soft coating and the stiff substrate, and facilitates the expansion of the crack expanding into the substrate. Whereas the one-layered coating can more effectively prevent the crack from propagating into the substrate than can the two- or four-layered coating. The investigation also indicates that the material gradations of S-FGMs influence SIFs obviously only when the crack tip is inside the coating that is stiffer than the substrate. As the crack extends through the coating and into the substrate, the material gradation of the S-FGM coating and the material mismatch of the multi-layered coating slightly bear on the values of SIF.     

Fracture mechanisms of ceramic coating during wear

A typical buckling phenomenon of the coating on the wear groove caused by the residual compressed stress was analyzed by the interface fracture mechanics and the buckling theory. It has been found that there is a critical thickness of coating on the wear groove for the buckling. The critical thickness can be calculated by t_b/c_d = [12(1 − v²_f)σ_R/π²E_f]1/2 (here t_b is the coating thickness, c_d the length of the interfacial crack, v_f the Poisson's ratio of the coating, σ_R the residual compressed stress in the coating, and E_f the elastic modulus of the coating).     

2024铝合金表面扫描式微弧氧化工艺研究

本研究利用小功率微弧氧化电源, 通过内充液式管状阴极的逐行扫描, 在2024铝合金样件表面生成微弧氧化陶瓷膜层, 对样件的局部受损部位进行了成功的修复, 从而突破了传统微弧氧化技术不能用于铝合金构件现场局部防护与修复的限制; 利用XRD、SEM、EDS等分析方法对陶瓷膜层的相组成与微观组织形貌进行了研究。利用纳米压痕仪测试了陶瓷膜层的纳米压痕硬度和弹性模量, 用动电位极化曲线测试陶瓷膜层的耐腐蚀性能。结果表明: 在恒电流模式下, 扫描式微弧氧化电压快速升高, 直接进入微弧放电阶段。其一次扫描成膜层厚度17 μm, 相对于传统微弧氧化具有很高的成膜效率。铝合金扫描式微弧氧化陶瓷膜层主要由α-Al₂O₃和γ-Al₂O₃组成, 膜层分为致密层和疏松层, 表面多微孔, 且有微裂纹; 纳米压痕测试结果表明, 陶瓷膜层纳米压痕硬度和弹性模量沿界面向外呈现先增加后减小的变化趋势。动电位极化曲线表明, 扫描式和传统微弧氧化陶瓷膜层都能够对基体起到有效的腐蚀防护作用, 传统微弧氧化陶瓷膜层的腐蚀防护作用高于扫描式。     

Fracture-resistant ultralloys for space-power applications: Normal-spectral-emissivity and electron-emission studies of tungsten, rhenium, hafnium-carbide alloys at elevated temperatures

A series in this journal on high-temperature properties of “fracture-resistant ultralloys for space-power systems” preceded the present paper: the antecedent publications covered tungsten(W), rhenium(Re) alloys with and without thoria(ThO₂) (W, 23Re; W, 27Re; W, 30Re and W, 30Re, 1ThO₂). This paper reports radiative and thermionic effects of hafnium carbide(HfC) and Re variation in W alloys: normal spectral emissivity(ε_λ) is used in pyrometry to determine the true temperature of a surface. Effective work function (φ_e) is an important consideration in the selection of the electrode materials for high-temperature thermionic energy converters in space-power applications. The _0.535μ, ε_0.65μ and φ_e trends of W, Re, 0.35HfC with 5–20% Re were measured in the range of 1700–2500K. The results indicate that ε_λ decreases with increasing temperatures and Re contents. The presence of HfC produced higher ε_λ values than those of sintered materials with comparable W,Re alloy contents. The results also indicate that φ_e increases with rhenium contents. This can be explained as growth of the potential barrier at the metal, vacuum boundary associated with a volume effect—the decrease in the lattice constant of W.     

Analysis of stiffness reduction of cracked cross-ply laminates

Stiffness reduction of cracked [0°_m/90°_n]_s laminates is analyzed by variational methods on the basis of the principle of minimum complementary energy. For this purpose admissible stress systems are constructed which satisfy equilibrium and all boundary and interface conditions. The optimal stress field is then determined by minimization of complementary energy. The analysis allows for crack interaction and random crack distribution. Results are given for Young's modulus, shear modulus and Poisson's ratio. Young's modulus results are in excellent agreement with experimental data for [[0°/90°₃]]_s glass/epoxy laminate.     

Laser processing of ZrO2 coatings

Potential materials for protective heat-resistant coatings are the so-called fragmentary porous ceramic layers penetrated by a net of microcracks. The fragments can be shifted easily during thermal cycling procedure, and the micro-cracks prevent the throughout crack propagation, which could destroy the coating. Laser surface processing of coatings is one of the effective ways to form the fragmentary layered structure.

The peculiarities of laser processing of ZrO₂+Y₂O₃ plasma sprayed coatings deposited onto the steel substrate with the Ni-Cr-Al-Y sub-layer alloy were investigated. The coatings were processed by CW CO₂ and Nd:YAG lasers (Fig. 1). The laser processing resulted in melting of the coating surface. The modified coating consisted of a number of macro fragments with sizes 200-500 μm and in turn they consisted of a number of micro-fragments with sizes 20-70 μm. Both types of the fragments are separated by wide (10-15 μm) or narrow (1-5 μm) cracks accordingly. The structure and some properties of the modified coatings such as heat-resistance, hardness, surface roughness, and tightness are investigated depending on the laser output parameters.     

C.V.D. Coating of Alumina Reinforcing Particles to Improve the Wear Resistance of H.S.S. Based Composites

C.V.D. coating of the reinforcing ceramic particles used in particulate metal matrix composites allows the control of reactivity at the particle/matrix interface. Wear resistant high speed steel-based composites containing uncoated A1₂0₃, uncoated TiC and C.V.D. coated A1₂0₃ were liquid phase sintered, then characterized using “pin-on-disc” wear testing. TiC or TiN C.V.D. coatings of A1₂0₃ were tested to determine die increase in reactivity of the particles with the liquid phases formed during sintering. This resulted in a porosity decrease at the particle/matrix interface in addition to a better ceramic/metal cohesion due to improved wettability. Reactivity and wettability were studied using differential thermal analysis, electron microprobe analysis, transmission electron microscopy, and image analysis. Results from pin-on-disc wear testing illustrated the role of the C.V.D. coating on the wear behavior of the studied materials. Lower wear rates were obtained with the composites containing TiC or TiN-coated Al₂0₃. These results showed that there is a relation between wettability of ceramic particles by the metallic phases and wear resistance of the composites.     

Residual stress, Young''s modulus and fracture stress of hot flame deposited diamond

Chemical vapour deposition (CVD) diamond coatings deposited on various substrates usually contain residual stresses. Since the residual stress affects the adhesion of the coating to the substrate, as well as the performance of the coating/substrate composite in many technical applications it is of importance to study the magnitude of these stresses.

In the present study the hot flame method was used to deposit diamond coatings on cemented carbide inserts by scanning the surface with a nine flame nozzle. By varying the oxygen to acetylene flow ratio and the deposition time coatings of different qualities and thicknesses were obtained. The residual strain/stress of the coatings was measured by three different methods: X-ray diffraction using the sin² (Ψ) method, Raman spectroscopy and disc deflection measurements. To extract the residual stress from the strain data the Young's modulus was obtained from bending tests of diamond cantilever beams manufactured from free standing diamond films. The latter technique was also used to determine the fracture stress of the diamond films.

All deposited coatings displayed a residual compressive strain/stress state. The residual strain in the diamond coatings did not vary with coating thickness (1.5 μm to 20 μm) but was found to increase from −1.8 × 10⁻³ to −2.2 × 10⁻³ with decreasing diamond quality. The compressive residual stress was found to decrease from −2 GPa to −1.3 GPa with decreasing diamond quality. This is mainly due to a decrease in Young's modulus (from 1.1 TPa to 0.6 TPa) with decreasing diamond quality. Also the fracture stress was found to decrease (from 1.8 GPa to 0.8 GPa) with decreasing diamond quality. The three methods used for measuring the stress state in the coatings, X-ray diffraction, Raman spectroscopy and deflection measurement, all give the same result. The deflection technique has the advantage that no information about the elastic properties of the coating is needed, whereas Raman spectroscopy has the best lateral resolution (≈5 μm) and is the fastest method (≈5 min).     

Highly conductive microcrystalline silicon carbide films deposited by the hot wire cell method and its application to amorphous silicon solar cells

Microcrystalline silicon carbide (μc-Si_1−xC_x) films were successfully deposited by the hot wire cell method using a gas mixture of SiH₄, H₂ and C₂H₂. It was confirmed by Fourier transform infrared and X-ray diffraction analyses that the films consisted of μc-Si grains embedded in a-Si_1−xC_x tissue. The p-type μc-Si_1−xC_x films were deposited using B₂H₆ as a doping gas. A dark conductivity of 0.2 S/cm and an activation energy of 0.067 eV were obtained. The p-type μc-Si_1−xC_x was used as a window layer of a-Si solar cells, in which the intrinsic layer was deposited by photo-chemical vapor deposition, and an initial conversion efficiency of 10.2% was obtained.     

Measurement of continuous damage parameter

The present paper introduces several measuring methods of continuous damage parameter derived by the classical definition D = 1 − (A_e/A_a): (1) The measuring method on the basis of D = 1 − (E'/ E); (2) The measuring method on the basis of D = 1 − (ε₁/ε₂); (3) The measuring method on the basis of D = −Δρ_o/ρ_o, (4) The measuring method on the basis of D = A_d/A_a, and comments on these measuring methods.     

锆合金表面Cr基涂层的耐高温氧化性能

用磁控溅射法在锆合金基体表面制备Cr和CrAl层,并使其在1200℃/1 h水蒸汽中氧化,用扫描电子显微镜(SEM)、能谱仪(EDS)和X射线衍射仪(XRD)等手段表征氧化前后涂层和Zr合金基体的微观结构,研究了两种涂层在(反应堆失水(LOCA)事故情况下的)高温蒸汽环境中的抗氧化性能。结果表明：在1200℃/1 h水蒸汽中氧化后没有涂层的锆合金基体表面生成厚度约为100 μm的氧化膜;而在Cr涂层表面生成的致密Cr₂O₃层其厚度约为4 μm,表明氧化速率显著降低。CrAl涂层氧化后表面生成致密的Cr₂O₃和Al₂O₃混合氧化层,其厚度只有0.8 μm,表明氧化速率进一步降低。这些结果表明: 用磁控溅射法在锆合金表面制备的Cr和CrAl涂层,在1200℃水蒸气环境中均表现出良好的耐氧化性能。在Cr涂层表面生成的氧化膜厚度约为未涂层锆合金氧化层的1/25,CrAl涂层氧化膜厚度低于锆合金表面氧化层的1/100。     

Self-catalytic degradation of iron-bearing chemical conversion coating on magnesium alloys ---- Influence of Fe content

A number of industrial and biomedical fields, such as hydraulic fracturing balls for gas and petroleum exploitation and implant materials, require Mg alloys with rapid dissolution. An iron-bearing phosphate chemical conversion (PCC) coating with self-catalytic degradation function was fabricated on the Mg alloy AZ31. Surface morphologies, chemical compositions and degradation behaviors of the PCC coating were investigated through FE-SEM, XPS, XRD, FTIR, electrochemical and hydrogen evolution tests. Results indicated that the PCC coating was characterized by iron, its phosphates and hydroxides, amorphous Mg(OH)₂ and Mg_3−n(H_nPO₄)₂. The self-catalytic degradation effects were predominately concerned with the Fe concentration, chemical composition and microstructure of the PCC coating, which were ascribed to the galvanic corrosion between Fe in the PCC coating and the Mg substrate. The coating with higher Fe content and porous microstructure exhibited a higher degradation rate than that of the AZ31 substrate, while the coating with a trace of Fe and compact surface disclosed a slightly enhanced corrosion resistance for the AZ31 substrate.     

Structure, nucleation and Eu luminescence in alkaline earth rare earth borate glass

In La₂O₃-MO-B₂O₃ ternary system, various glasses/ glass ceramics with M=Ca, Sr and Ba have been prepared. In this ternary system, homogeneous nucleation occuring in the B₂O₃ melt appears to be the cause for the formation of nanocrystallites, hence glass ceramics. The nucleation process is very much dependent on the alkaline earth used viz., with smaller alkaline earths like Ca, boron prefers (BO₄)⁵⁻ tetrahedral coordination with oxygen, while in the case of bigger alkaline earths like Ba and Sr, (BO₃)³⁻ triangular coordination seems to be predominant. Eu³⁺ in this glass system yields intense ⁵D₀→⁷F_j emission. A cursory view on the dependence of the various Judd-Ofelt parameters (Ω_k) indicates that Ω₂ parameter is very much dependent on the immediate vicinity of the luminescent ion (Eu³⁺) while Ω₄ is not. Various results based on these are discussed.     

Influence of stress ratio at baseline loading on delayed retardation phenomena of fatigue crack growth in A553 steel and A5083 aluminium alloy

The delayed retardation phenomena of fatigue crack growth following a single application of tensile overload were investigated under the baseline loading with the stress ratio, R = σ_min/σ_max, ranging from −1 to 0.5 for A553 steel and A5083 aluminium alloy. Two different overload cycles were applied; the one is the case that the ratio of peak stress range to baseline stress range, r = Δσ₂/Δσ₁, is equal to two and the other is the case that the ratio of maximum peak stress to maximum baseline stress, σ_2max/σ_1max, is equal to two. The retardation took place stronger in aluminium than in steel. Under the condition of r = 2 the normalized number of cycles, N_D/N_C, (N_D: the number of cycles during retardation, N_C: the number of cycles required for propagation through the overload-affected-zone size) decreased slightly as the R ratio increased from −1 to 0.5, while under the condition of σ_2max/σ_1max = 2 the N_D/N_C-values increased drastically as the R ratio increased from −1 to 0 (or the overload ratio, r, increased from 1.5 to 2) in both the materials. These retardation behaviors were expressed theoretically according to the model proposed by Matsuoka and Tanaka [1, 3] by using four parameters: the overload ratio, r, the exponent in Paris equation, m, the overload-affected-zone size, ω_D, and the distance at the inflection point, ω_B.     

Atmospheric plasma discharge chemical vapor deposition of SnOx thin films using various tin precursors

A series of 0.2–0.6 μm thick SnO_x films were deposited onto borosilicate and sodalime silica glass substrates by atmospheric plasma discharge chemical vapor deposition at 80 °C. SnO_x films deposited from monobutyltin trichloride contained a large percentage of SnCl₂:2H₂O, and therefore were partially soluble in water. SnO_x coatings deposited from tetrabutyltin were not soluble in water or organic solvents, had good adhesion even at growth rates as high as 2.3 nm/s, had high transparency of  90% and electrical resistivity of 10⁷ Ω cm. As-grown tin oxide coatings were amorphous with a small concentration of SnO₂, SnO and Sn crystalline phases as determined by grazing angle X-ray diffraction and X-ray photoelectron spectroscopy measurements. Upon annealing in air at 600 °C the resistivity of SnO_x films decreased to 5–7 Ω cm. Furthermore, optical and X-ray measurements indicated that SnO_x was converted into SnO₂ (cassiterite) with a direct band gap of 3.66 eV. Annealing of as-grown SnO_x films in vacuum at 340 °C led to formation of the p-type conductor SnO/SnO_x. The indirect band gap of SnO was calculated from the optical spectra to be 0.3 eV.     

The relation between compressive strength of carbon/epoxy fabrics and micro-tow geometry with various bias angles

In this paper compressive tests of carbon/epoxy (plain weave, 3k) fabric composites were performed to investigate the relation between compressive strength and various bias angles and shear angles. Compressive properties such as chord modulus and maximum strength of the fabric composite materials are essential to analyze the drape behaviour and estimate the quality of the final products. Various specimens with different bias and shear angles which were fabricated by using autoclave de-gassing moulding process were prepared to estimate the strength and chord modulus with respect to the bias and shear angle variations. The stacking sequences of the compressive test specimens are [0]_10T, [15]_10T, [30]_10T and [45]_10T for bias specimens and [±37]_10T, [±32]_10T, [±28]_10T, [±22]_10T for sheared specimens. Micro-tow structures were observed to correlate the fabric compressive strength with crimp angle. Anti-buckling rig was involved to prevent specimens from buckling during the compressive tests. The compressive test was performed with 1.3 mm/min strain rates. Compressive test results were compared with calculation results. Facture modes which were classified in two different modes were analyzed using microscopic observation.     

Impact fatigue properties of epoxy resin filled with SiO2 particles

Impact fatigue tests were carried out on epoxy resin filled with SiO₂ particles. The effects of the percentage of SiO₂ particles and the impact cyclic loading frequency on the impact fatigue strength was investigated. The micromechanism of impact fatigue failure was examined and correlated with the morphology of the fracture surface. The impact stress amplitude, σ_t, can be estimated by the formula, σ₂(N_f · Te)^mt = D_t where (N_f· Te) is the cumulative duration time, and mt and D_t, are parameters describing impact fatigue characteristics. The impact fatigue strength and the static strength are governed by the percent of SiO₂ particles. Crack initiation under monotonie cyclic impact loading was attributed to decision of the epoxy-SiO₂ interface. Unstable crack propagation occurs when the crack passes through the SiO₂ particles.     

Incorporating Runs Rules into Hotelling's χ2 Control Charts

This paper illustrates a simple and effective method of incorporating runs rules into Hotelling χ² control charts. A Markov chain will be used to obtain a desired in-control average run length (ARL). Comparisons between the basic multivariate χ² control chart and the multivariate χ² control chart, which incorporates the various runs rules, are based on their respective ARL performances. All multivariate χ² control charts that incorporate the various runs rules have shown better ARL performance compared to the basic multivariate χ² control chart for small shifts in distance λ from the in-control mean vector μ₀ to the out-of-control mean vector μ_s. An example of the application, based on the proposed method, is also given.     

Ag glasses based on Zn-phosphate

The partial substitution of Zn²⁺ for Ag⁺ in Ag₄P₂O₇ leads to the formation of a wide glassy domain of composition [Ag₄P₂O₇] _(1−y) [Zn₂P₂O₇] _(y) with 0.20y0.87. The introduction of AgI in these materials results in a new series of glasses of formula [(Ag₄P₂O₇)_(1−y) (Zn₂P₂O₇)_(y)] _(1−X) [AgI] _(x), which domain for the composition y = 0.25 corresponds to 0x 0.64. The structure as well as the thermal and electrical properties of these materials are compared with those of the [AgPO₃] _(1−X) [AgI] _(x) and [Ag₄P₂O₇] _(1−x) [AgI] _(x) glasses.