Jianning Ding

Mechanical properties and fracture toughness of multilayer hard coatings using nanoindentation

Abstract The nanoindentation fracture of multilayer hard coatings of TiN/Ti(C,N)/TiC, Al 2 O 3 /TiC/Ti(C,N)/TiC, TiN/Ti(C,N)/TiC/Ti(C,N)/TiC and TiN/Ti(C,N)/TiC/Ti(C,N)/TiC/Ti(C,N)/TiC, deposited on cemented carbide using a CVD technique was studied. Based on an analysis of the energy released in cracking, the calculated fracture toughness values of these coatings were 2.18, 1.74, 3.40 and 3.90 MPa m 1/2 , respectively. Both load–penetration depth and load–penetration depth squared plots have been demonstrated to be necessary if a more complete understanding of the coating system behaviour is to be gained. The interfacial failure and the critical load of interfacial failure of these coatings were also discussed. A step was found in the force–displacement curves at the onset of coating fracture. A straight line segment in the load–penetration depth squared curves was discussed. The hardness, fracture toughness and anti-wearability of these coatings were compared. The results show that the fracture toughness and anti-wearability are getting higher with the layers increasing.

Specimen size effect on mechanical properties of polysilicon microcantilever beams measured by deflection using a nanoindenter

Abstract The validity of a novel, direct and convenient method for micromechanical property measurements by beam bending using a nanoindenter has been demonstrated. In the deflection of microbeams, the influence of the indenter tip pushing into the top of the microbeams and the curvature across its width must be considered. The elastic deflection of a polysilicon microcantilever beam will vary linearly with the force. Thus, Youngs modulus of the beam determined from the slope of this linear relation is 156 Gpa±2.9–6.3%. The size effects of the manufactured component on Youngs modulus and strength of polysilicon microcantilever beams were measured. This result shows that the rupture strengths of polysilicon beams show size effect on the effective volume and surface area of the specimens. The rupture strength of each sample decreased with increasing the specimen effective volume and surface area, but increased with increasing the surface-to-volume ratio. The defect that initiates fracture is often on the surface of the specimen. In such cases the size effect can be traced back to a surface-to-volume ratio as the governing parameter.

Theoretical study of the sticking of a membrane strip in MEMS under the Casimir effect

Mechanical stability and sticking are troublesome problems in microfabrication and operation processes when the separations of components in microelectromechanical systems are in the sub-micrometre regime. Some hitherto neglected mechanical effects, including the quantum mechanical effect, should be taken into account for solving the problems. The magnitude of the Casimir force is significant when the membranes work in vacuum without the effect of capillary forces. In this paper, an analysis is presented of the influence of the Casimir effect with surface roughness, conductivity and temperature corrected on the deformation of a membrane strip structure. With nothing other than the Casimir force loading the strip, a stable static equilibrium state and an unstable static equilibrium state exist, depending on the value of a dimensionless constant K. The membrane strip will collapse if the value of K is larger than the critical value of KC. The critical value of KC varies with the value of w0. This provides a way to check if a system with given dimensions and material properties will be in a stable equilibrium. This also provides a way of designing a membrane strip with high resistance to collapse.

Size effect on the mechanical properties of microfabricated polysilicon thin films

A new microtensile test device using a magnetic-solenoid force actuator was developed to evaluate the mechanical properties of microfabricated polysilicon thin films that were 100–660 mm long, 20–200 μm wide, and 2.4-μm thick. It was found that the measured average value of Youngs modulus, 164 GPa ± 1.2 GPa, falls within the theoretical bounds. The average fracture strength is 1.36 GPa with a standard deviation of 0.14 GPa, and the Weibull modulus is 10.4–11.7. Statistical analysis of the specimen size effects on the tensile strength predicated the size effects on the length, the surface area, and the volume of the specimens. The fracture strength increases with an increase of the ratio of surface area to volume. In such cases, the size effect can be corrected to the ratio of the surface area to volume as the governing parameter. The test data accounts for the uncertainties in mechanical properties and may be used to enhance the reliability and design of polysilicon microelectromechanical systems devices.

Size effect on the mechanical properties and reliability analysis of microfabricated polysilicon thin films

The increasing use of microelectromechanical systems (MEMS) and advanced sensors has led to concern about the failure modes and reliability of these structures. The wider acceptance of MEMS devices depends on the solution of issues associated with materials, design, and fabrication. The reliable mechanical properties of thin films are critical to the safety and functioning of these complex microdevices and should be accurately determined. In order to determine mechanical properties for reliable MEMS design, a new microtensile test device using a magnetic-solenoid force actuator was developed. Mechanical properties of microfabricated polysilicon thin films with dimensions of 100 to 660 /spl mu/m long, 20 to 200 /spl mu/m wide, and 2.4 /spl mu/m thick were evaluated. It was found that the average value of Youngs modulus, 164 GPa/spl plusmn/1.2 GPa, falls within the theoretical bounds. The average fracture strength was 1.36 GPa with a standard deviation of 0.14 GPa, and the Weibull moduli were between 10.4 and 11.7. Statistical analysis of tensile strength for various specimen sizes predicted the effects of specimen length, surface area, and volume that occurs due to microstructural and dimensional constraints. The test data accounts for the uncertainties in mechanical properties of miniature elements and may be used for design of a reliable polysilicon MEMS. Based on the test results of 40 specimens to failure, it is recommended to maintain nominal strain below 0.0057 /spl mu/m//spl mu/m for a safe design.

Scale dependence of tensile strength of micromachined polysilicon MEMS structures due to microstructural and dimensional constraints

The success of microelectromechanical systems (MEMS) as a key technology in the 21st century depends in no small part on the solution of materials issues associated with the design and fabrication of complex MEMS devices. The reliable mechanical properties of these thin films are critical to the safety and functioning of these microdevices and should be accurately determined. In order to accomplish a reliable mechanical design of MEMS, a new microtensile test device using a magnetic-solenoid force actuator was developed to evaluate the mechanical properties of microfabricated polysilicon thin films with dimensions of 100–660 μm length, 20–200 μm width, and 2.4 μm thickness. It was found that the measured average value of Young’s modulus, 164 ±1.2 GPa, falls within the theoretical bounds. The average fracture strength is 1.36 GPa with a standard deviation of 0.14 GPa, and the Weibull modulus is 10.4–11.7, respectively. Statistical analysis of the specimen size effect on the tensile strength predicated the size effect on the length, the surface area and the volume of the specimens due to microstructural and dimensional constraints. The fracture strength increases with the increase of the ratio of surface area to volume. In such cases the size effect can be traced back to the ratio of surface area to volume as the governing parameter. The test data account for the uncertainties in mechanical properties and may be used in the future reliability design of polysilicon MEMS. The testing of 40 specimens to failure results in a recommendation for design that the nominal strain be maintained below 0.0057.

The tribological properties of YBa2Cu3O7 films in ambient environment

Abstract In the present study, high-Tc superconducting thin YBa2Cu3O7 films and polysilicon films were prepared to investigate the initial sliding friction properties using a ball-on-flat tribometer when samples were moved against a sapphire ball or a steel ball in ambient environment. The surface topography was measured with atomic force microscope (AFM). After five times testing, the experimental results indicate that the friction coefficient of YBa2Cu3O7 films is lower than that of polysilicon films when sliding against a sapphire ball and almost the same when sliding against a steel ball. In particular, the initial friction of YBa2Cu3O7 films is more stable when sliding against a sapphire ball. However, the initial friction of polysilicon films fluctuates during a cycle period when sliding against a sapphire ball. They are both stable when sliding against a steel ball. Although, the surface profile of the YBa2Cu3O7 film is rough and can be seen to be rougher than the polysilicon film, but the friction coefficient of the YBa2Cu3O7 film is lower than that of polysilicon film. Also, although the topography of YBa2Cu3O7 films changes during friction, the friction coefficients are stable. This clearly shows that the initial sliding friction of YBa2Cu3O7 films under microfriction is stable. The observation signifies YBCO film is a good film to prevent stick–slip motion in ambient environment. The wear properties of YBa2Cu3O7 films suggest that the superconducting outgrowths (CuO) are loose and they can be easily removed.

Mechanical Stability and Sticking in a Model Microelectromechanical Systems (MEMS) under Casimir Forces–Part I: Corrections to the Casimir Force

The sizes of and the separations between the components in some MEMS are already in the sub-micrometer regime, where some quantum mechanical effects, hitherto neglected, will need to be taken into account, for example, the Casimir effect. In this paper, the roughness, conductivity and temperature corrections to the Casimir force theoretical simulations between two polysilicon parallel plates were considered in detail. The theoretical results show that if the effects of the roughness, conductivity, and temperature are combined together, the maximum error within 1 μ m of Casimir force per unit area between two parallel polysilicon plates is 0.262. What this means is that the surface roughness and finite conductivity corrections should be taken into account in precision Casimir force simulation within small space separations.

Mechanical Stability and Sticking in a Model Microelectromechanical systems (MEMS) under Casimir Forces–Part II: The Role of the Casimir Effect

Mechanical stability and sticking are the troublesome problems in the microfabrication process and operating when components in MEMS working in the sub-micrometer regime. In this paper, the analysis of the role of the Casimir effect on a membrane strip structure is presented. With nothing other than the Casimir force loading the strip, there exist a stable static equilibrium state and an unstable static equilibrium state. The state can be determined by the value of dimensionless constant K. The membrane strip will collapse if Κ is larger than the critical value of Kc. The critical value of Kc varies with the value of w0. This provides a way to check if a system of given dimensions and material properties will have a stable equilibrium position in the absence of other forces, such as an electrostatic actuation force, or a capillary force during and after the wet etching of a sacrificial layer. This also provides a way to design a membrane strip with high aspect ratio (L/h) that is not easily to collapse into the surface S.