铸造碳化钨颗粒增强PDC钻头胎体的三体磨损行为

采用无压浸渗工艺制备了铸造碳化钨颗粒增强PDC钻头胎体材料,胎体材料组织均匀,胎体中碳化钨颗粒完整,碳化钨颗粒与铜合金基体形成均匀扩散层。重点研究了PDC钻头胎体的三体磨料磨损行为和磨损机理。结果表明:铸造碳化钨颗粒形貌是影响PDC钻头胎体三体磨料磨损行为的主要因素。相对于破碎铸造碳化钨,球形碳化钨内部微裂纹少且无应力集中,具有耐磨增效作用,可显著提高PDC钻头胎体材料的三体磨损性能。球形碳化钨颗粒增强PDC钻头胎体的相对耐磨性是破碎碳化钨颗粒增强PDC钻头胎体的10倍。破碎碳化钨颗粒增强PDC钻头胎体的磨损表面呈现大量铜合金基体犁沟,多角状碳化钨颗粒被磨损变圆滑;而球形碳化钨颗粒增强PDC钻头胎体的磨损表面碳化钨颗粒突出林立,少量碳化钨颗粒被折断或发生破裂。     

Optical intensities of Pr ions in transparent oxyfluoride glass and glass–ceramic. Applications of the standard and modified Judd–Ofelt theories

The optical characterisation of Pr³⁺ ions in transparent SiO₂–Al₂O₃–CdF₂–PbF₂–YF₃ based glass and glass–ceramic have been performed. From absorption and emission spectra the oscillator strengths of the 4f²–4f² electronic transitions have been obtained. The intensity parameters have been calculated using both the Judd–Ofelt theory and the modified theory developed by Kornienko, Kaminskii and Dunina. A comparison of the experimental oscillator strengths, the spontaneous emission probabilities and the lifetimes of the ³P₀ level and those calculated using the above theoretical procedures has been performed for both samples. The root mean square deviation found using the standard Judd–Ofelt theory is larger than the value obtained with the modified treatment.     

增强颗粒对镁基复合材料磨损性能的影响(英文)

研究增强颗粒Mg2Si对镁基复合材料摩擦磨损性能的影响,讨论Si加入量、载荷和滑动速度对Mg2Si/AM60镁基复合材料磨损性能的影响。结果表明,向镁合金中加入合金元素Si,可原位生成增强颗粒Mg2Si,增强颗粒Mg2Si可明显提高AM60镁合金的磨损性能。随着载荷和滑动速度的增加,AM60镁合金和Mg2Si/AM60镁基复合材料的磨损量都增大。AM60镁合金的磨损机制为粘着磨损。随着载荷的增大,Mg2Si/AM60镁基复合材料的磨损由磨粒磨损向粘着磨损转变。     

High stress abrasive wear behavior of sillimanite-reinforced Al-alloy matrix composite: A factorial design approach

An attempt has been made to explore the possibility of using a natural mineral, namely sillimanite, as dispersoid for synthesizing aluminum alloy composite by solidification technique. The abrasive wear behavior of this composite has been studied through factorial design of experiments. The wear behavior of the composite (Y _composite) and the alloy (Y _alloy) is expressed in terms of the coded values of different experimental parameters like applied load (x ₁), abrasive size (x ₂), and sliding distance (x ₃) by the following linear regression equations:

Extrusion of spray-formed Al–25Si–X composites and their evaluation

A comprehensive methodology to fabricate an Al–25Si–X composite via spray-forming and extrusion was investigated to judge the potential of this alloy for applications requiring high strength as well as high wear resistance and/or high elastic modulus. Billets were spray-formed with a low gas/metal ratio of about 0.8 m³/kg to reduce the consumption of nitrogen gas. The billets, having considerable porosity, were then extruded into sub-scale bars using various extrusion parameters, such as extrusion ratio, die temperature, and die configuration to optimize the extrusion conditions and eliminate the pores in the preform. Based on results obtained from the preliminary sub-scale extrusion tests, the spray-formed billets were then hot extruded into a tubular shape with outer and inner diameters of 97 mm × 77 mm, respectively, at an extrusion temperature of 500 °C. Various material properties of the extruded tubes were measured and compared with other candidate materials for these applications.     

Microstructural characterization and abrasive wear performance of HVOF sprayed Cr3C2–NiCr coating

Cr₃C₂–NiCr coatings were deposited by high velocity oxy-fuel (HVOF) process with different spray parameters to examine dominant microstructural factors in abrasive wear of the coatings. The microstructure of the HVOF sprayed Cr₃C₂–NiCr coatings was characterized by scanning electron microscopy and transmission electron microscopy (TEM). The apparent average size and volume fraction of carbide particles in the coatings were estimated through a quantitative imaging analysis. The formation of carbide phases in the coating was discussed based on the TEM observation results. The abrasive wear behavior of the coating was evaluated by the dry rubber wheel abrasive wear test and the wear mechanisms were elucidated. Influences of apparent size and volume fraction of carbide particles on the abrasive wear weight loss were examined through correlating the proposed relation with the experimental results. Results showed that Cr₃C₂ particle size was significantly reduced after the spraying and Cr₇C₃ carbide was present around Cr₃C₂ particles, and Cr₂₃C₆ carbide was dispersed in NiCr alloy matrix with a nano-crystalline structure. The three carbides were formed in the coating through different mechanisms. The removal of carbide particles in the coating was mainly responsible for the abrasive wear of the coating. The content and particle size of the Cr₃C₂ carbides were the two key factors controlling the abrasive wear of the HVOF sprayed Cr₃C₂–NiCr coatings.     

Taguchi approach to influence of processing parameters onerosive wear behaviour of Al7034-T6 composites

Taguchi technique was used to predict the influence of processing parameters on the erosive wear behavior Al7034-T6 composite reinforced with SiC and Al₂O₃ particles in different mass fractions. These hybrid metal matrix composites (HMMCs) were fabricated by using a simple technique called stir casting technique. Scanning electron microscope (SEM) was used to study the surface morphology of the composite and its evolution according to processing time. The design of experiment (DOE) based on Taguchi's L₁₆ orthogonal array was used to identify various erosion trials. The most influencing parameter affecting the wear rate was identified. The results indicate that erosion wear rate of this hybrid composite is greatly influenced more by filler content and impact velocity respectively compared to other factors. This also shows the significant wear resistance with the increase in the filler contents of SiC and Al₂O₃ particles, respectively.     

Laser cladding of stainless steel with Ni–Cr3C2 and Ni–WC for improving erosive–corrosive wear performance

The microstructure and the erosive–corrosive wear (ECW) performance of laser-clad Ni–Cr₃C₂ and Ni–WC coatings with overlapping clad tracks (OCT) on a 0.2% C martensitic stainless steel were investigated by scanning electron microscopy (SEM), XRD, EDX techniques and ECW testing. The coating produced by completely dissolving Cr₃C₂ particles in laser melted pool is composed of austenite (γ) dendrites surrounded by a γ-M₇C₃ eutectic, whereas another one is of granular solidifying structure in which contains the incompletely dissolved WC particles. The microhardness of Ni–WC coating is higher than that of Ni–Cr₃C₂, about 300 HV average. The main reason of microhardness difference is that two coatings have different solidified structure. The comparison of ECW tests found that the reduction of ECW rate dose not occur with the increase of hardness. The Ni–Cr₃C₂ coating with lower hardness has a lower ECW rate with respect to the Ni–WC one. Both average ECW rate decreased by approximately 30% and 60% as compared to that of stainless steel substrate, and both coatings had different ECW mechanism. The increase of ECW resistance is closely related to structure state, kind and amount of carbides, microhardness and toughening ability of the clad layer.     

Influence of aluminum oxides on abrasive wear resistance of Fe–50 at.% Al intermetallic sinters

This paper presents the influences of processing parameters on the tribological properties of Fe–50 at.% Al intermetallics obtained by sintering in the presence of liquid phase elemental Fe and Al powders. The wear resistance of FeAl under dry sliding against C45QT steel under ambient conditions was studied in a pin-on-disc contact configuration. Although processing parameters such as sintering temperature and homogenization time had an effect on the wear resistance of FeAl intermetallics, the particle size of Al₂O₃ oxides derived from oxidized aluminum and iron powders and formed during sintering had the greatest influence.     

多壁纳米碳管／Cu基复合材料的摩擦磨损特性

利用销－盘式磨损试验机研究了粉末冶金法制备的多壁纳米碳管／Cu基复合材料的稳态摩擦磨损行为,并用扫描电镜分析了复合材料的磨损形貌。结果表明：多壁纳米碳管／Cu基复合材料具有较小的摩擦系数,并随纳米碳管质量分数的增加而逐渐降低;由于复合材料中纳米碳管的增强和减摩作用,在低载荷和中等载荷作用下,随着纳米碳管质量分数的增加,复合材料的磨损率减小;而在高载荷作用下,由于发生表面开裂和片状层剥落,含纳米碳管质量分数高的复合材料的磨损率增高。     

Performance and wear characteristics of ceramic, cemented carbide, and metal nozzles used in coal–water–slurry boilers

Ceramics, cemented carbides, and metals were prepared to be used as nozzles in CWS boilers. CWS burning tests in a boiler with these nozzles were carried out. The erosion wear resistance of these nozzles was compared by determining their erosion rates and hole diameter variation. Results showed that the life of the ceramic nozzles is about 30 times high than that of the metal nozzles. The wear types at the nozzle wall surface differed in various positions. The nozzle center wall section suffers form abrasive impact under low impact angles, and the damage at the center wall mainly occurs by plowing and plastic deformation for metals, and by polishing action for carbides and ceramics. The primary wear mechanisms at the exit of ceramic nozzle exhibited thermal shock damage with chipping owing to the greater thermal stresses.     

Preparation of MoB and MoB–MoSi2 composites by combustion synthesis in SHS mode

Combustion synthesis in the mode of self-propagating high-temperature synthesis (SHS) was carried out in the Mo–B and Mo–B–Si systems for the preparation of molybdenum boride MoB and the composite of MoB–MoSi₂ from elemental powder compacts. Under a preheating temperature above 150 °C, the reaction of Mo with boron in the sample compact of Mo:B = 1:1 is characterized by a planar combustion front propagating in a self-sustaining and steady manner. As the preheating temperature or sample compaction density increased, combustion temperature was found to increase and the propagation rate of the combustion front was correspondingly enhanced. Moreover, the XRD analysis provides evidence of yielding nearly single-phase -MoB from the Mo–B sample at equiatomic stoichiometry. In the synthesis of MoB–MoSi₂ composites, the starting stoichiometry of the Mo–B–Si powder compact was varied so as to produce the final composites containing 20–80 mol% MoB. It was also found the increase of flame-front velocity and combustion temperature with increasing MoB content formed in the composite. The composition analysis by XRD shows excellent conversion from the Mo–B–Si powder compact to the MoB–MoSi₂ composite through the SHS reaction; that is, in addition to a small amount of Mo₅Si₃, the as-synthesized composite is composed entirely of MoB and MoSi₂.     

An FEM investigation into the behavior of metal matrix composites: Tool–particle interaction during orthogonal cutting

An analytical or experimental method is often unable to explore the behavior of a metal matrix composite (MMC) during machining due to the complex deformation and interactions among particles, tool and matrix. This paper investigates the matrix deformation and tool–particle interactions during machining using the finite element method. Based on the geometrical orientations, the interaction between tool and particle reinforcements was categorized into three scenarios: particles along, above and below the cutting path. The development of stress and strain fields in the MMC was analyzed and physical phenomena such as tool wear, particle debonding, displacements and inhomogeneous deformation of matrix material were explored. It was found that tool–particle interaction and stress/strain distributions in the particles/matrix are responsible for particle debonding, surface damage and tool wear during machining of MMC.     

Tool wear and machining performance of cBN–TiN coated carbide inserts and PCBN compact inserts in turning AISI 4340 hardened steel

PCBN is the dominant tool material for hard turning applications due to its high hardness, high wear resistance, and high thermal stability. However, the inflexibility of fabricating PCBN inserts with complex tool geometries and the prohibitive cost of PCBN inserts are some of the concerns in furthering the implementation of CBN based materials for hard turning. In this paper, we present the results of a thorough investigation of cBN plus TiN (cBN–TiN) composite-coated, commercial grade, carbide inserts (CNMA 432, WC–Co (6% Co)) for hard turning applications in an effort to address these concerns. The effect of cutting speed and feed rate on tool wear (tool life), surface roughness, and cutting forces of the cBN–TiN coated carbide inserts was experimented and analyzed using analysis of variance (ANOVA) technique, and the cutting conditions for their maximum tool life were evaluated. The tool wear, surface roughness, and cutting forces of the cBN–TiN coated and commercially available PCBN tipped inserts were compared under similar cutting conditions. Both flank wear and crater wear were observed. The flank wear is mainly due to abrasive actions of the martensite present in the hardened AISI 4340 alloy. The crater wear of the cBN–TiN coated inserts is less than that of the PCBN inserts because of the lubricity of TiN capping layer on the cBN–TiN coating. The coated CNMA 432 inserts produce a good surface finish (<1.6 μm) and yield a tool life of about 18 min per cutting edge. In addition, cost analysis based on total machining cost per part was performed for the comparison of the economic viability between the cBN–TiN coated and PCBN inserts.     

Temperature dependent conductivity and thermoelectric power of the iodine doped poly(vinyl alcohol)–Cu chelate

We have investigated the temperature dependence of electrical conductivity and thermoelectric power (TEP) at 1.7 K < T < 300 K in an organo metallic complex, the iodine doped poly(vinyl alcohol)–Cu²⁺ chelate. We observed intrinsic metallic temperature dependence of resistivity from room temperature to 68 K with a broad minimum [ρ(68 K)/ρ(300 K) 0.75], which has not been observed previously in similar organo metallic complexes. There occurs an unusual metal-insulator transition at T 68 K and the resisitivity increases upon cooling below 68 K. However, the low temperature resistivity becomes finite (instead of going to infinity), [ρ(1.7 K)/ρ(300 K) 0.98] indicating that a quantum mechanical tunneling conduction is dominant at this low temperature. It is remarkable that the resistivity at 1.7 K is as small as that of room temperature. Such unusual temperature dependence of conductivity could be understood as thermally assisted hopping conduction between metallic islands. However, the observed intrinsic metallic temperature dependence of resistivity implies that such hopping conduction barrier is not important at high temperature (T > 68 K). The intrinsic metallic characteristics are confirmed by the quasi-linear temperature dependence of TEP for the whole measured temperature range (1.7 K < T < 300 K) with a small slope change at low temperature, T < 68 K, which is understood as an effect of variable range hopping (VRH) conduction at low temperature. The results of magneto resistance (MR) and magneto thermoelectric power (MTEP) are consistent with the above interpretation.     

Fabrication and dry sliding wear behavior of in situ Al–K2ZrF6–KBF4 composites reinforced by Al3Zr and ZrB2 particles

Aluminum matrix composites reinforced by Al₃Zr and ZrB₂ particles were fabricated from Al–x wt.%(K₂ZrF₆–KBF₄) (x = 5, 10, 15, 20, 25) systems via magnetochemistry in situ reaction and the dry sliding wear properties and behavior of the composites were investigated. XRD and SEM analysis show that ZrB₂ and Al₃Zr reinforcement phases have been obtained and been distributed uniformly in the aluminum matrix. Wear test results show that the values of wear weight loss of the composites decrease with the increase of x under all identical wear conditions, and that of the relative wear resistance R_relat. increases under the applied load of 100 N. Especially, when x = 25, the wear weight loss (under a sliding time of 120 min and an applied load of 100 N), which is 0.245 to that of the A356 alloy, and the R_relat. (under the intermediate wear-sliding stage and an applied load of 100 N) is 4.772, which is 1.513 to that of the primary stage, respectively. Two modes of the wear mechanisms of the as-prepared composites were identified.     

The effect of sliding speed and amount of loading on friction and wear behavior of Cu–0.65 wt.%Cr alloy

Friction and wear behavior of a peak aged Cu–0.65 wt.%Cr alloy was investigated. The friction and wear experiments were run under ambient conditions with a pin-on-disk tribometer. Experiments were performed using various applied normal loads and sliding velocities. The tribological behavior of the studied alloy was discussed in terms of friction coefficient, wear loss and wear mechanism.Friction coefficient and wear loss have shown large sensitivity to the applied normal load and the sliding velocity. At the sliding velocity of 0.3 m/s weight loss increased from 6.9 to 51 mg by increasing the normal load from 20 to 40 N. At higher sliding velocity minimum weight loss is achieved at 60 N normal load. So it can be seen that with increasing normal load wear rate decreases due to the formation of a continuous tribofilm which consists of Fe–Cu intermetallic. Varying of friction coefficients in different conditions of normal load and sliding velocity is correlated to the wear behavior.The analysis of worn surfaces by XRD and SEM showed that an increase in normal load and sliding velocity creates an intermetallic wear-induced layer, which modifies the wear behavior of the alloy. The XRD result indicates that new phase of Cu_9.9Fe_0.1 is generated on worn surfaces of the pin specimens during the wear tests. There is a significant correlation between the micrograph of worn surfaces and the wear rate of specimens.     

Modeling the compressive strength of molasses–cement sand system using design of experiments and back propagation neural network

Molasses, an eco-friendly and relatively cheap binder may be used as a substitute for chemical binders. For commercial exploitation of the molasses–cement sand system it is essential to generate models for predicting the properties of the sand mix from the composition. Central composite design is used to develop regression equations for predicting compressive strength of the sand mix when molasses is varied between 5.5% and 7.5% and cement between 2% and 4%. Though central composite design is an effective tool for studying the complex effects of number of independent variables on response factor it has quite a few limitations. Back propagation neural network is not only capable of modeling highly non-linear relationship using dispersed data in the solution domain but has a few advantages over the central composite design. But one of the major drawbacks of this network is that no theoretical basis exists to determine the number of hidden layers and number of neurons therein. Different configurations of BPNN have great effects on the predicted results. Back propagation neural networks of different configurations are trained. Results obtained form these networks are analyzed and compared with those obtained form regression equations and experiments. Guidelines for selecting the effective configuration of back propagation networks are proposed.