“擦擦”造像艺术的功能性特征研究

“擦擦”一词是藏语对模制泥佛像梵语的音译。据《元史·释老志》记载“有作“擦擦”者.以泥作小浮屠也。”可见“擦擦”是佛、菩萨等造像的缩微小像.虽然擦擦是缩微小像,但是极其微妙和传神.造像不论从造型到题材.都严格遵照“三经一疏”的仪轨程序来进行,是藏传佛教造像艺术中最具朴素语言和平民特质的圣物。

“擦擦”

“擦擦”，是梵语的音译，亦称脱模泥塑，是藏传佛教和藏族民间艺术的共同形式。其制作方法是用泥塑手法造型后，用铜、铁、木、陶等制作成印模，在模内填充胶泥，经过挤压、脱模制作而成。“擦擦”常作为佛塔塔肚内的填充圣物，或作为修炼、还愿的功德资粮、或作为虔诚的偶像神灵崇拜物，是藏族雕塑中独树异帜，别具一格的微塑艺术品类，在藏区朝佛圣地随处可见，应用极广。

西藏西部的古格王国及其“擦擦”泥塑艺术

藏传佛教文物以唯美的艺术造型、精良的制作工艺和深厚的文化底蕴而著称,获得了国际上诸多学术领域的广泛关注。为深入了解其艺术价值,考察西藏西部地区古格王国的政治文化背景,以时代顺序为主线,对具有明确发现地点、经科学发掘的'擦擦'泥塑标准器进行文化和艺术元素探索,参考其它公私收藏的泥塑以艺术品,文字叙述和文化内涵分析,力图多方位解读古格王国'擦擦'的艺术特点。

吐蕃印章演化及文化特性初探

西藏印章起源于何时在中外印学研究中始终是聚讼未了的棘手难题。目前学界多采用“述而不作”,不下结论的稳妥办法,介绍西藏古代印章则从出土或传世的吐蕃时期印章说起。“擦擦”印模自传入西藏以来,其复制功能、护佑功能和传播功能对吐蕃印章的起源、发展、演变以及文化特性、风格特点等均产生了广泛而深刻的影响。尤其是吐蕃印章以佛、苯教中最具代表性、象征性的图文符号作为钮式或印纹内容承载着护佑教法、护佑政权、护佑安康之意。通过“擦擦”印模与早期吐蕃印章实物在初始功能、阴刻方式上的一致性以及对钮制造型、印面图像、钤盖载体、使用范围等方面表现出的诸多文化共性的对比分析,可以得知西藏印章起源脱胎于4-5世纪的“擦擦”印模。

Friction heat production and atom diffusion behaviors during Mg-Ti rotating friction welding process

An innovative physical simulation apparatus, including high speed camera, red thermal imaging system, and mechanical quantity sensor, was used to investigate the friction heat generation and atom diffusion behavior during Mg-Ti friction welding process. The results show that the friction coefficient mainly experiences two steady stages. The first steady stage corresponds to the Coulomb friction with material abrasion. The second steady stage corresponds to the stick friction with fully plastic flow. Moreover, the increasing rates of axial displacement, temperature and friction coefficient are obviously enhanced with the increase of rotation speed and axial pressure. It can also be found that the there exists rapid diffusion phenomenon in the Mg-Ti friction welding system. The large deformation activated diffusion coefficient is about 105 higher than that activated by thermal.

Advance and trend of friction study in plastic forming

Friction is a critical issue in plastic forming which influences forming force, metal flow, forming quality and service life of die. Since friction is a highly nonlinear physical phenomenon which is interactively affected by so many factors, great efforts have been made to study the friction mechanism and controlling. The research progress of friction issues in plastic forming was summarized and discussed from four aspects： testing, characterizing, modeling and optimization/controlling. Considering urgent demands for green, efficient and precise forming of high-performance, lightweight and complex components in high-tech industries such as aerospace and automotive, the trends and challenges of friction study in plastic forming were proposed.

Friction and wear properties of in-situ synthesized Al_2O_3 reinforced aluminum composites

Al-5%Si-AI2O3 composites were prepared by powder metallurgy and in-situ reactive synthesis technology. Friction and wear properties of Al-5%Si-Al2O3 composites were studied using an M-2000 wear tester. The effects of load, sliding speed and long time continuous friction on friction and wear properties of Al-5%Si-Al2O3 composites were investigated, respectively. Wear surface and wear mechanism of Al-5%Si-Al2O3 composites were studied by Quanta 200 FE-SEM. Results showed that with load increasing, wear loss and coefficient of friction increased. With sliding speed going up, the surface temperature of sample made the rate of the producing of oxidation layer increase, while wear loss and coefficient of friction decreased. With the sliding distance increasing, coefficient of friction increased because the adhesive wear mechanism occurred in the initial stage, then formation and destruction of the oxide layer on the surface of the sample tended to a dynamic equilibrium, the surface state of the sample was relatively stable and so did the coefficient of friction. The experiment shows that the main wear mechanism of Al-5%Si-Al2O3 composites includes abrasive wear, adhesive wear and oxidation wear.

Preparation and tribological performances of Ni-P-multi-walled carbon nanotubes composite coatings

Multi-walled carbon nanotubes （MWNTs） were wet-milled in the presence of ammonia and cationic surfactant and then used as reinforcements to prepare Ni-P-MWNTs composite coatings by electroless plating. The tribological performances of the composite coatings under dry condition were investigated in comparison with 45 steel and conventional Ni-P coating, Micrographs show that short MWNTs with uniform length and open tips were obtained through the wet-milling process. The results of wear test reveal that the Ni-P-MWNTs composite coatings posses much better friction reduction and anti-wear performances when compared with 45 steel and Ni-P coating. Within the MWNTs content range of 0.74%-1.97%, the friction coefficient and the volume wear rate of the composite coatings decrease gradually and reach the minimum values of 0.08 and 6.22x10-15 m3/（N.m）, respectively. The excellent tribological performances of the composite coatings can be attributed to the introduction of MWNTs, which play both roles of reinforcements and solid lubricant during the wear process.

Joint torque-based Cartesian impedance control with friction compensations

In order to investigate the joint torque-based Cartesian impedance control strategies and the influence of compensations for friction, an experimental study on the identification of friction parameters, friction compensation and the Cartesian impedance control are developed for the harmonic drive robot, by using the sensors available in the joint itself. Different from the conventional Cartesian impedance control schemes which are mostly based on the robot end force/torque information, five joint torque-based Cartesian impedance control schemes are considered, including the force-based schemes in Cartesian/joint space, the position-based schemes in Cartesian/joint space and the stiffness control. Four of them are verified by corresponding experiments with/without friction compensations. By comparison, it is found that the force-based impedance control strategy is more suitable than the position-based one for the robot based on joint torque feedback and the friction has even a positive effect on Cartesian impedance control stability.

OBSERVER/FILTER STRUCTURE BASED ADAPTIVE FRICTION COMPENSATION FOR HIGH PRECISION TURNTABLE

Two adaptive friction compensation schemes are developed for a high precision turntable system with nonlinear dynamic friction to handle two types of parametric uncertainties in the friction. Both schemes utilize a nonlinear observer/filter structure to compensate for uncertainties in corresponding friction parameters associated with the turntable system. Moreover, in the second scheme, adjustable gains are introduced into the dual nonlin- ear filters and they can be tuned to improve the position tracking performance. In both cases, a Lyapunov-like argument is provided for the global asymptotic stability of the closed-loop system. Simulation results demonstrate the effectiveness of the proposed schemes.

Dynamic engagement characteristics of wet clutch based on hydro-mechanical continuously variable transmission

The effect of the design parameter on the clutch engagement process of the hydro-mechanical continuously variable transmission(CVT)was investigated.First,the model of the power train was developed with the software of SimulationX,and the clutch shift experiment was used to validate the correctness of the model.Then,the friction coefficient function was fitted with the test data to get the friction coefficient model suitable for this paper.Finally,based on the evaluating index of the friction torque and the friction power,two groups of design parameters(oil pressure and friction coefficient)were simulated and explained the changing regulation theoretically.According to the simulation results,the high oil pressure and friction coefficient can reduce the slipping time.The large oil pressure can increase the peak torque but the effect of friction coefficient on the peak torque is not so significant.The friction power reaches the maximum value at 3.2 s,the peak value increases as the oil pressure and friction coefficient increase.The effect of the oil pressure on the clutch engagement and thermal performance is greater than the friction coefficient.

Friction properties of groove texture on Cr12MoV surface

To analyze the influence of surface texture on friction properties of Crl2MoV＇, ordinary grinder and spinning technology were adopted to obtain the grooved surface morphology of samples, and then the impact of spindle speed and feed in z-direction on surface morphology in the process of spinning was studied. In addition, the corresponding friction coefficient of sample was obtained through friction and wear tests. The results show that the peak clipping and the valley filling were conducted on the grinding surface, which could improve the surface roughness effectively and make the grinding trench-type wear scar more uniform. Both the area ratio of groove and groove spacing increased initially and then decreased with the increase of the spindle speed or the feed in z-direction. As a kind of micro-process, the groove could influence the friction coefficient of sample surface, whose distribution was beneficial to the reduction of friction coefficient. Compared with the surface obtained through ordinary grinding, grooved surface morphology through spinning technology was more conductive to reduce the friction coefficient, which could be reduced by 25%. When the friction coefficient of sample was reduced to the minimum, the texture of groove corresponded had an optimal area ratio and an optimal groove spacing, 37.5% and 27.5 μm, respectively.

Microstructural zones and tensile characteristics of friction stir welded joint of TC4 titanium alloy

TC4 titanium alloy was friction stir welded using a W-Re pin tool,and the defect-free weld was produced with proper welding parameters.The joint consists of stir zone,heat affected zone and base material.The stir zone is characterized by equiaxed dynamically recrystallized α phases and transformed β phases with fine α+β lamellar microstructure.The microstructure of the heat-affected zone is similar to that of the base material,but there is an increase in the volume fraction of β.Transverse tensile strength of the joint is 92% that of the base material,and the joint is fractured in the stir zone and the fracture surface possesses typical plastic fracture characteristics.The stir zone is the weakest part of the joint,through which the tensile characteristics of the TC4 joint can be explained.

Effect of zinc powder content on tribological behaviors of brake friction materials

Brake friction materials with different zinc powder contents(0,2,4,6,8 wt.%)were fabricated via powder metallurgy method.The results indicate that with the increasing zinc powder content,the density and thermal conductivity of the materials gradually increase,while the hardness decreases monotonously.With increasing zinc powder content,the curve of the nominal friction coefficient shows fluctuating trend but the lowest friction coefficient also shows an increase.However,the wear rate and braking noise of the friction material monotonously decrease with increasing zinc content.This effect may be attributed to the transformation of the tribological mechanism from adhesive wear and abrasive wear to adhesive wear.The brake friction material with 4 wt.%zinc powder exhibits both the best tribological and noise performance.

Tribological properties of Fe(Cr)-B alloys at high temperature

This work is aimed to study the effect of boron on wear resistance of Fe-Cr-B alloys containing different boron contents(0 wt%,5 wt%,7 wt%and 9 wt%)from room temperature(RT)to 800°C in order to explore their applications as high-temperature wear resistant mechanical parts.Additionally,the wear mechanism of alloys is evaluated.The tribological properties of alloys are systematically studied by using a ball-on-disc tribometer at 10 N and 0.20 m/s from RT to 800°C sliding against Si3N4 ceramic ball.The boron element greatly improves the wear resistance of specimens as compared with that of unreinforced specimen.The friction coefficients of specimens decrease with increasing of testing temperature.The wear rates of Fe-Cr-B alloys decrease firstly and then raise with the increase of boron content.The specific wear rates of specimens with boron are 1/10 of the unreinforced specimen.Fe-21wt%Cr-7wt%B keeps the best tribological properties at high temperature.

Dry sliding wear behavior of Al_2O_3 fiber and SiC particle reinforced aluminium based MMCs fabricated by squeeze casting method

Al2O3 fiber （Al2O3f） and SiC particle （SiCp） hybrid metal matrix composites （MMCs） were fabricated by squeeze casting method.The tests were carried out using a pin-on-disk friction and wear tester by sliding these pin specimens at a constant speed of 0.36 m/s （570 r/min） against a steel counter disk at room temperature,100 C and 150 C,respectively.To observe the wear characteristics and investigate the wear mechanism,the morphologies of the worn surfaces and specific wear rate were analyzed by using scanning electron microscope （SEM） and Arrhenius plots.Moreover,the effects of fiber orientation and hybrid ratio were discussed.

Effects of sliding velocity and normal load on tribological behavior of aged Al-Sn-Cu alloy

The tribological behavior of aged Al-Sn-Cu alloy rubbed in the presence of lubricant over a range of sliding velocities and normal loads was investigated. The results showed that peak-aged （PA） alloy had a better tribological behavior than under-aged （UA） and over-aged （OA） alloys, which could be attributed to the optimized strength-ductility matching and a better hardness under PA condition. Wear rate and friction coefficient showed great sensitivity to applied sliding velocity and normal load. The wear rate and friction coefficient of the alloy exhibited a reduction trend with the increase in sliding velocity. The low wear rate and friction coefficient of alloy at high velocities were due to the effectively protected film and homogeneous Sn on surface. However, an increase in normal load led to an obvious increment in wear rate. The friction coefficient exhibited a fluctuant trend with the increase of normal loads. The seriously destroyed film and abraded Sn resulted in poor tribological behavior at high normal loads. The Sn particles and lubricant film which includes low shear interfacial lubricating layer and oxide tribolayer are the key to the tribological behavior of Al-Sn-Cu alloy.

Effect of Fe-impurity on tribological properties of Al-15Mg_2Si composite

The effect of Fe-impurity（0.2%-2%, mass fraction） on the microstructure, dry sliding wear, and friction properties of Al-15 Mg2 Si composite was investigated using a pin-on-disk tester under the applied pressures of 0.25, 0.5 and 1 MPa at a constant sliding speed of 0.13 m/s. According to the results, Fe modified the primary Mg2 Si particles from irregular dendritic form to smaller particles with polyhedral shapes, refined the pseudo-eutectic structure, and led to the formation of hard b-Al5 Fe Si platelets in the matrix. In spite of hardness improvement by these microstructural changes, the resistance of the composite against dry sliding wear was impaired. SEM examination of the worn surfaces, wear debris, and subsurface regions confirmed the negative effect of b-phase on the tribological properties. It was found that b-particles were fractured easily, thereby decreasing the potential of the substrate to resist against sliding stresses and giving rise to the instability and easy detachment of tribolayer as large delaminated debris. The friction results also revealed that Fe slightly decreased the average friction coefficient, but increased the fluctuation in friction.