微小尺度下金属变形与疲劳研究

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"金属的疲劳是材料科学中的一个重要主题，特别是在设计和制造机械设备时。这篇由T. CONNOLLEY, P.E. MCHUGH和M. BRUZZI撰写的综述文章深入探讨了在微小尺度下金属的变形与疲劳现象。文章发表在2005年的《Journal of Materials Science & Technology》上，doi为10.1111/j.1460-2695.2005.00951.x。随着科技的进步，机械装置的尺寸已经进入纳米和微米级别，远小于传统的力学测试样品。这些微小设备的出现提出了如何预测其机械性能和可靠性的挑战。常规的宏观测试数据可能不再适用，因为这些数据基于的样品尺寸远大于实际设备，可能导致性能预测的不准确。因此，研究小型化样品和设备的测试方法变得至关重要。文章主要集中在自由站立的金属样品上的小尺度测试，旨在揭示尺寸效应如何影响金属的疲劳特性。尺寸效应是指当材料尺寸减小时，其物理和力学性质可能发生显著变化的现象。例如，在微观和纳米尺度下，金属的晶界、缺陷以及表面粗糙度等特征会更加显著地影响其疲劳寿命和失效模式。疲劳过程通常包括应力诱导的循环塑性变形，随着时间的推移导致材料的裂纹形成和扩展，最终导致结构的突然失效。在小尺度下，这些过程可能会加速，因为表面和体积比例的改变可以增加应力集中，降低材料的疲劳极限。此外，微小尺寸下的热输运和应变硬化行为也可能有所不同，这些因素都对疲劳性能有重大影响。文章详细讨论了实验技术，如电子显微镜下的原位测试，以观察和理解微小尺度下金属的变形机制。同时，作者也探讨了理论模型和模拟方法，以更准确地预测微纳米尺度金属部件的疲劳行为。这些研究对于开发新的材料和设计更可靠的微型机械系统具有重要意义。总结来说，这篇综述提供了对微小尺度金属疲劳现象的深入理解，强调了尺寸效应在预测和控制这些材料性能时的重要性。这对于工程师和研究人员来说是一个宝贵的资源，有助于他们在设计和优化纳米及微米级别的机械设备时考虑到这些关键因素，从而提高设备的性能和可靠性。"

DEFORMATION AND FATIGUE OF METALS AT SMALL SIZE SCALES 1125

components. The same manufacturing method as that

used for actual components should also be used. Major

challenges in characterising the mechanical properties of

small components are measuring the small forces and dis-

placements involved.

Mechanical property characterization involves the fol-

lowing issues:

Specimen design & manufacturing.

Gripping & force application.

Displacement or strain measurement.

Some of the micro-scale mechanical test methods (ten-

sion test, axisymmetric bend test, microbeam bend test,

bulge test, M-test, wafer curvature test, resonance tests,

passive strain sensors & Raman spectroscopy) were re-

cently reviewed by Srikar et al.

They proposed a rational

approach to selecting the test techniques required for the

design of different types of microsystem as follows:

Characterize the structure of interest.

Identify the mechanical properties of interest, including

residual stresses.

Select suitable test samples, bearing in mind that the time

and costs involved in specimen preparation are generally

much longer than for the actual mechanical testing. Where

possible, aim for simpler test samples that can be fabricated

in arrays or batches.

If more than one test method is suitable, narrow the choice

by considering the ease of instrumentation and gripping

and ease of data interpretation.

MEMS research has sparked an incredible burst of cre-

ativity in the design of devices and the methods used to

test them. This raises the familiar issue of standardization,

and whether or not results obtained in different labora-

tories are directly comparable. Tsuchiya et al.

organized

a round-robin test at five different Japanese institutions,

to tensile test single crystal silicon, polysilicon, thin film

nickel and thin film titanium. The methods used were:

1 On-chip tensile testing method with a motorized microm-

eter actuator.

2 A ‘palm top’ tester with a mechanical grip.

3 Motorized micrometer system with a mechanical clamp.

4 Piezo actuator with an electrostatic grip.

5 Magnetostrictive actuator with a micro-glued grip.

The main difference in techniques was the method of

gripping the samples. All five systems used image analysis

for strain measurement. Specimens of each material were

fabricated on a single wafer so the processing parameters

were the same. Even so, three different specimen designs

were required to fit the different test machines. The sin-

gle crystal & polycrystal silicon tests showed linear elas-

tic behaviour and stress–strain curves obtained from each

lab were similar. The nickel specimens had low ductility,

breaking soon after yield. There was a large difference

in the curve slopes and strains to failure for different labs.

The largest strains were observed in tests with lower load-

ing rates, illustrating how test conditions like strain rate

can influence the observed properties. For titanium, re-

sults were similar for all labs, but problems of specimen

damage were encountered with the mechanical gripping

methods.

In addition to mechanical testing, material characteri-

zation is required to understand structure-property re-

lationships, identify the cause of failure and explain any

observed size effects. Desirable information includes:

Microstructure (grain size, porosity, distribution of flaws

and inclusions, dislocation density).

Surface roughness and topography.

Environmental interactions (corrosion, response to tem-

perature).

Fractographic information.

So, before comparing test results from different research

groups, it must be remembered that materials processing

parameters, device fabrication parameters and test proce-

dures vary from laboratory to laboratory, even for what is

ostensibly the same material (for example LIGA nickel).

This is understandable, given the flexibility and creativity

enabled by micro-fabrication techniques. In the follow-

ing sections, mechanical test results for different types of

metallic structure are reviewed, with the aim of summaris-

ing the range of information available.

Monotonic deformation

Monotonic mechanical properties are required so that

the desired deflection or deformation of micro-scale de-

vices can be predicted. For devices that operate in the

elastic regime, knowledge of the size effects on elastic

modulus and yield strength is required. In other devices,

controlled yielding may be utilized as part of a deploy-

ment process, so sufficient ductility and tensile strength is

necessary to avoid premature fracture. Knowledge of

monotonic stress–strain behaviour is also important for

determining the level of in-service stresses that could

cause fatigue. The available monotonic data for foils and

films are summarized in Table 1, which shows that no

study has fully characterized trends in tensile properties

(elastic modulus, proof stress, ultimate tensile strength

and elongation) as a function of thickness, specimen size

and microstructure. Similar data for wires are summarized

in Table 2.



2005 Blackwell Publishing Ltd.

Fatigue Fract Engng Mater Struct

28, 1119–1152

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