Measurement of Interfacial Mechanical Properties in Fiber-Reinforced Ceramic Composites

Properties of the fiber/matrix interface in a SiC/glass-ceramic composite were investigated using an indentation method in Which a pyramidal indenter is used to push on the fibers and cause sliding at the interface. An ultralow-load indentation instrument was used to measure force and displacement continuously during loading, unloading, and load cycling. Frictional sliding and combined debonding/frictional sliding at the interface were analyzed. The analyses enabled the results to be used to provide a measure of the debond fracture energy, the magnitude of the frictional sliding stress, a measure of the uniformity of the frictional stress, and an indication of the sensitivity of the frictional stress to repeated sliding, varying load rate, and exposure to high temperatures.     

Indentation responses of a graded zirconium phosphate–filled epoxy resin

The effects of load and time on the Vickers indentation responses of a graded zirconium phosphate (ZP)–filled epoxy resin are described. The hardness of this material is dependent on the concentration of ZP dispersed within the epoxy matrix. In the region poor in ZP, the hardness response is independent of load. In contrast, the hardness response in the region rich in ZP is profoundly load‐dependent as a combined result of particle agglomeration and an indentation‐size effect. When compared with the ZP‐rich‐epoxy, the ZP‐poor epoxy exhibits a larger creep and a more pronounced elastic recovery in the Vickers impression. The nature and degree of deformation in the vicinity of Vickers contacts are also studied. During indentation the ZP‐rich epoxy exhibits no contact‐induced cracks but displays microscale plasticity, which can be associated with intergrain sliding, debonding, and grain push‐out. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 931–935, 2001     

Indentation Fatigue in Random and Textured Alumina Composites

The effect of grain orientation on contact fatigue behavior has been investigated by using alumina with elongated grains as a model system. Two kinds of composite microstructures, textured and random, were prepared by controlling the processing conditions. The textured material has the platelets aligned parallel to the surface, and the random material has the platelets randomly oriented. The Hertzian indentation results show that, although both materials exhibit damage accumulation with increasing number of cycles due to frictional degradation at the microstructural level, the damage evolution rate is much lower for the textured material. This suppression of fatigue damage in the textured material appears to result from the lower shear stress concentration along the textured weak interfaces between elongated alumina grains. The implication of the present results for structural design in improvement of contact-fatigue resistance is also addressed.     

A "Partitioned-Problem" Approach to Microstructural Modeling of a Glass-Ceramic

The indentation response of a mica-containing glass-ceramic is studied. In this type of material, Hertzian cone cracks, which normally occur in brittle materials that have been loaded with a hard spherical indenter, are suppressed in favor of distributed subsurface damage, which is indicative of yield. Theoretical analysis, finite-element modeling, and experimental results are used to establish a connection between the macroscopic behavior of the material and events that occur on a microstructural scale. This association is achieved by first determining the macroscopic material properties, such as the yield stress and the strain-hardening index, in terms of the microstructure. Then, these properties are used to predict the macroscopic indentation response of the material. Thus, the problem is partitioned into microstructural and macroscopic domains. This work is particularly relevant to the design of structural ceramics in machining, wear, and bearings applications.     

Estimation of Interfacial Shear in Ceramic Composites from Frictional Heating Measurements

A new approach for estimating the interfacial frictional shear stress in fiber-reinforced ceramics is presented. The approach is based upon measurement of the temperature rise which occurs during the cyclic loading of ceramic composites. This temperature rise, which is caused by the frictional slip of fibers within the composite, is related to the level of frictional shear stress which exists along debonded interfaces. To illustrate the technique, the interfacial shear stress in a unidirectional Nicalon-fiber calcium aluminosilicate matrix composite was determined at ambient temperature.     

Contact/scratch-induced surface deformation and damage of copper-graphite particulate composites

The elastoplastic surface deformation and damage under frictional sliding contact of copper-graphite particulate composites with Cu-content ranging from 0 to 40 vol% are examined in indentation and scratch tests using a load controlled test system. The contact areas in indentation and scratch tests are estimated with the Field-Swain approximation. The characteristic material parameters of the elastic modulus E′, yield stress Y, interfacial shear strength s, and the scratch resistance are discussed in relation to the Cu-content of the composites. The microscopic mechanisms and processes for the surface deformation and damage induced by frictional sliding contact are also examined. With the increase in the normal contact load, the scratch-induced surface deformation and damage are transiently followed with the sequential four stages: (I) the elastoplastic grooving, (II) plastic plowing, (III) microcracking, and (IV) the inter- and intra-fractures and chipping of graphite particles. The Cu-content in the composite plays the key role in controlling the characteristic contact pressures for these transitional deformation/damage processes.     

Stress–Strain Behavior of Nicalon-Fiber-Reinforced Calcium Aluminosilicate Composites under Tensile Fatigue Conditions

The unusual stress–strain hysteresis loop shape exhibited by ceramic-matrix composites under cyclic loading has previously been explained as a result of either strain rate dependence of the frictional shear stress or crack closure. This investigation has determined that the response is due to neither mechanism. Instead, it is proposed that a variation of interfacial shear strength occurs during each cycle. A static coefficient of friction dominates immediately after loading or unloading. A much lower dynamic coefficient of friction operates once fiber sliding begins. This dynamic coefficient appears to be very dependent on surface roughness.     

The determination of the coefficient of dynamic friction of organic powder compacts on steel

A practical experimental model system has been successfully used to study the frictional response of organic powder compacts sliding across a polished steel plate, representative of the die bore of a production compaction system. This system offers a controlled approach to the study of frictional phenomena occurring during compaction and facilitates a more detailed investigation into the fundamental mechanisms of friction than a simple resolution of forces within a punch and die apparatus.For acetylsalicyclic acid sliding on steel, the dynamic friction coefficient was found to be dependent on the displacement and, to a lesser extent, the initial normal load, whereas for PTFE on steel, the dynamic friction coefficient was independent of displacement and load and estimated at 0.09. Thus, soft organic materials exhibit very different frictional characteristics to those of brittle materials. These differences reflect the differing importance and magnitude of the three frictional components, adhesion, shear and ploughing.     

Effects of water molecules on tribological behavior and property measurements in nano-indentation processes - a numerical analysis

Nano/micro-manufacturing under wet condition is an important consideration for various tool-based processes such as indentation, scratching, and machining. The existence of liquids adds complexity to the system, changes the tool/work interfacial condition, and affects material behaviors. For indentation, it may also affect material property measurements. However, little effort has been made to study this challenging issue at nano- or atomistic scale. In this study, we tackle this challenge by investigating nano-indentation processes submerged in water using the molecular dynamics (MD) simulation approach. Compared with dry indentation in which no water molecules are present, the existence of water molecules causes the increase of indentation force in initial penetration, but the decrease of indentation force in full penetration. It also reduces the sticking phenomenon between the work and tool atoms during indenter retraction, such that the indentation geometry can be better retained. Meanwhile, nano-indentation under wet condition exhibits the indentation size effect, while dry nano-indentation exhibits the reverse indentation size effect. The existence of water leads to higher computed hardness values at low indentation loads and a smaller value of Young''s modulus. In addition, the friction along the tool/work interface is significantly reduced under wet indentation.     

Constitutive response of calcium‐silicate‐hydrate layers under combined loading

Calcium‐silicate‐hydrate (C‐S‐H) is the main hydration product for ordinary Portland cement (OPC) materials that exhibits a layered structure containing interfaces that controls the system response to shear deformation at the nanometer scale. In this work, we used molecular statics simulations to study the mechanical behavior of an atomistic model of C‐S‐H under combined loading conditions that are typical of structural applications of these materials. Combined loading is implemented by first compressing or stretching the atomistic structure to impose an external hydrostatic pressure, and then loading the system through both heterogeneous and homogeneous shear deformation. By utilizing two different shear methodologies, we were able to isolate the interface behavior from the bulk response. Our results show several qualitative similarities with that of macroscale cementitious materials including pressure sensitivity of the maximum shear strength and strength asymmetry in compression and tension. This indicates that the well‐known cohesive‐frictional behavior of cementitious materials is fundamental to interfaces between C‐S‐H grains at the nanoscale. Comparing differences in our results with nanoindentation experiments motivate future investigations of the effect of C‐S‐H particle size and morphology on strength scaling properties at the mesoscale. These mesoscale model interactions should include the normal‐stress or pressure dependency that we observe.     

Fracture resistance of dental glass-ceramics under sliding contact

Aesthetic glass-ceramics are widely employed dental materials, both in bulk form and as veneers. As they are highly brittle, the durability of these materials is limited by fracture and wear processes originated from contacts with opposing dentition and/or third-body particles during mastication and bruxism. This work investigates the resistance to fracture of commercial dental glass-ceramics under sliding contact, simulated by means of scratch tests. It finds that materials with relatively larger crystals (feldspathic and leucite) require a lower stress to fracture upon sliding than lithium silicates containing smaller (but more elongated) crystals, due to their larger defects and lower toughness. Results are analyzed as a function of material microstructure within the framework of Weibull theory and fracture mechanics. Implications for materials selection and development in prosthetic dentistry are briefly discussed.     

Nano-Scale Indentation Creep Testing at Non-Ambient Temperature

Depth sensing nanoindentation can be used to study the time-dependent deformation of very small volumes of materials, contacts, and thin films. Force modulation provides a continuous measure of the contact stiffness during an indentation, and minimises the adverse effect of thermal drift which is particularly important for sub-micron samples. Most of the nanoindentation experimental work so far has been carried out at room temperature. In this paper we describe a solid-state thermoelectric heating and cooling system which gives a straightforward way to vary the temperature of both sample and tip. The capabilities of the technique are demonstrated by observing the time and temperature dependent creep properties of high purity Indium. Hardness, its strain rate dependence, the stress exponent, and the activation energy for the creep process can all be directly measured from nanometre scale contacts, and the values obtained are similar to those from bulk conventional creep testing. The technique is likely to be of particular value for polymer thin films.     

Inelastic Deformation Mechanisms and Damage in Structural Ceramics Subjected to High-Velocity Impact

The inelastic deformation mechanisms and damage features observed in structural ceramics subjected to nonpenetrating, high-velocity impacts are similar to those seen in quasistatic Hertzian indentation, albeit more severe. For impacts on large ceramic bodies (relative to impactor diameter), cone cracking is the primary mechanism in regions of high tensile stresses. In regions of nonhydrostatic compressive stresses, depending on the material characteristics, elasticity, grain-boundary microcracking, or plasticity are the primary mechanisms, and depending on their associated energetics, may be able to compete with the initiation and growth of cone cracks. In this regard, a new model is presented that examines the effect of grain-boundary microcracking on cone cracking through shear-induced dilatancy (i.e., bulking) within the quasiplastic zone that forms just underneath the impact site. Depending on the size of the quasiplastic zone and bulking pressure, it is shown that the bulking phenomenon has the potential to suppress cone cracking. Lastly, examples of other shear-driven inelastic deformation mechanisms are presented.     

Interfacial Sliding Friction in Silicon Carbide–Borosilicate Glass Composites: A Comparison of Pullout and Pushout Tests

Interfacial sliding friction stress (τ_f) was assessed using both pushout and pullout tests on SiC-borosilicate glass composite specimens. Single-filament composite specimens were fabricated by heating to 950°C in argon borosilicate glass rods with fine-diameter (250-μm) capillary in which SiC filaments were inserted. The composite specimens prepared in this manner showed only frictional bonding. The maximum frictional sliding loads for pushout and the initial frictional sliding loads in pullout were measured as functions of the embedded length of the filament in the glass rods. The nonlinear variations of the frictional loads were analyzed using shear-lag models that include corrections for the effects of Poisson expansion or contraction on the sliding friction stress. It is shown that under identical conditions of composite fabrication the two tests give nearly identical properties for the interfaces. Pushout tests on hotpressed bulk composite specimens, however, showed both chemical bonding and a higher sliding friction stress relative to the single-filament capillary specimens. The presence of compressive residual stress on the filaments was independently confirmed by evidence of stress-induced birefringence.     

Shear Damage Mechanisms in a Woven, Nicalon-Reinforced Ceramic-Matrix Composite

The shear response of a Nicalon-reinforced ceramic-matrix composite was investigated using Iosipescu tests. Damage was characterized by X-ray, optical, and SEM techniques. The large inelastic strains which were observed were attributed to rigid body sliding of longitudinal blocks of material. These blocks are created by the development and extension of intralaminar cracks and ply delaminations. This research reveals that the debonding and sliding characteristics of the fiber—matrix interface control the shear strength, strain softening, and cyclic degradation of the material.     

Shear Properties of Thin Polymeric Films

When a thin solid organic coating is interposed between two contacting and sliding surfaces we may define a strength property τ as the frictional force per unit area of solid-solid contact. This paper reviews recent work on the influence of contact pressure, temperature and sliding velocity on τ for a range of high molecular weight organic polymers and lower molecular weight organic solids. It is shown that the shear properties of these thin films resemble those of the corresponding bulk polymers if allowance is made for the high degree of molecular orientation produced in the film during sliding.     

A Study of the Pull‐Out Performance of PPTA Fibres in Composites of Ethylene‐Type Ionomer Matrices/PPTA Fibres

The mean frictional shear stresses of six ionomer resins and sized Kevlar fibre were determined from fibre pull‐out tests. A study of the failure mechanisms occurring during pull‐out revealed that fibre delamination and fibre resin adhesion were factors which increased the measured frictional shear stresses and that there was a definite grouping of high and low frictional shear stress values. The low frictional shear stress values were used to calculate the mean frictional shear stress values, τ_B, because these were uncomplicated by fibre delamination and fibre resin adhesion, since these factors (delamination and adhesion) are certainly not unexpected in an ionomer/Kevlar composite. From these shear stress values, it was determined that critical fibre lengths should be between 35 and 72 mm for the high tensile strength Kevlar fibres within an ionomer matrix, for the composite to be used effectively. The ratio of the debonding force (F_B) to the frictional shear force (F_F), θ, did not vary significantly with the lengths of the embedded reinforcing fibres. Both debonding and frictional forces indicate increasing trends with the interfacial contact areas. The ratio of the interfacial bonding strength (τ_B) to the frictional shear stress (τ_F), ϕ, for the resin PEA‐6 compared to the surface modified poly(p‐phenylene terephthalamide) (PPTA) fibre ranged from 2 to 24. These ratios were grouped into two, viz: those where ϕ > 11 and those with ϕ < 7. Using only the τ_F where ϕ > 11 provided a mean frictional shear stress of 0.94 MPa and a standard deviation, s, of 0.23 MPa (the number of test samples, n, was 9). This value is little different from the frictional shear stresses measured for sized PPTA (0.84 MPa). The decrease in the values of ϕ is attributed to the decrease in τ_B, due to the surface modification reaction, without necessarily affecting the frictional shear stress, τ_F.     

Theory and Analysis of Fracture Energy in Fiber Reinforced Composites

A micro-mechanics model is developed to analyze the stress distributions and fracture energies associated with crack propagation and fiber pull-out in reinforced composites. The stress and work mechanisms of interfacial debonding, fiber deformation, and the frictional work of fiber pull-out are considered as semi-independent contributions to fracture toughness. The theoretical expressions of Cottrell for frictional work W_F and Outwater and Murphy for fiber deformational work W_D are obtained as special relations in a general relation for the total work W_T = W_s + W_F + W_D where W_s defines the matrix shear work for interfacial debonding of fiber and matrix. Three dimensional diagrams of fracture energies W_T, W_s, or W_r versus interfacial shear bond strength λ₀ and frictional shear stress λ_f identify regions of optimized fracture energy. The influence of environmental degradation of bond strength upon fracture energy is analyzed in terms of the theory.     

Cyclic Fatigue from Frictional Degradation at Bridging Grains in Alumina

Tension—tension cyclic loading tests have been conducted on a coarse-grained alumina ceramic that exhibits toughnesscurve behavior by grain-interlock bridging. Fatigue effects are observed in the regions of both short cracks, using indentation flaws, and long cracks, using compact-tension specimens. A true mechanical fatigue effect is demonstrated by running the tests below the static fatigue limit. A custom-made device for in situ observation of crack propagation in the scanning electron microscope enables us to identify bridge degradation as a cause of the fatigue process. "Wear" debris cumulates at the sliding intergranular frictional contact points, indicating a loss of traction at the junction. The basis of a fracture mechanics model describing the effect of this frictional degradation in reducing crack-tip shielding is outlined and fitted to the data. It is suggested that the bridge degradation fatigue mechanism may be widespread in polycrystalline ceramics with pronounced toughness curves.     

An experimental study on shear stress characteristics of polymers in plasticating single‐screw extruders

Frictional forces (for temperatures less than the melting or devitrification temperature) and viscous forces (for higher temperatures) have important roles on solids conveying and melting processes in plasticating single‐screw extruders. These forces are related to the shear stresses at polymer–metal interfaces. For temperatures at which the frictional forces are the main factor for the shear stresses, it is experimentally difficult to obtain the shear stresses at the polymer–metal interface. The interpretation of the data has further complications due to the frictional energy dissipation at the polymer–metal interface. An instrument called the Screw Simulator was used for further understanding of shear stresses at the polymer–metal interface and comparison of melting fluxes of different resins. This article presents the shear stress and melting flux measurements for low density polyethylene (LDPE), linear LDPE (LLDPE), acrylonitrile butadiene styrene (ABS), and high‐impact polystyrene (HIPS) resins as a function of sliding velocity and interface temperature at a fixed pressure of 0.7 MPa. The relationship between the experimental data and the extrusion process is also discussed. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers