Proper Selection Of Process Parameters Research Articles

Thermomechanical Properties during Cure of Glass-Polyester RTM Composites: Elastic and Viscoelastic Modeling

Resin transfer molding (RTM) is a widely used technique for the manufacturing of composite parts. A proper selection of process parameters is the key to yield successful molding results and obtain a good part. During composite consolidation, resin cure, also called chemical conversion, plays a decisive role on the final mechanical properties of the part. The modeling of resin kinetics and the evolution of composite properties during cure are crucial for process optimization. In this paper, the curing of a thermosetting polyester resin is studied by differential scanning calorimetry (DSC). A semiempirical autocatalytic model is developed to describe the kinetics of the chemical reaction. The model accounts for the maximum degree of polymerization as a function of cure temperature and induction time, i.e., the time required to attain total inhibitor degradation. The evolution of mechanical properties during resin cure for two glass-polyester composites is also studied with a dynamical mechanical thermal analyzer (DMTA) and a thermomechanical analyzer (TMA). Given that for a low chemical conversion, the elastic properties of the resin remain low, an initial degree of polymerization called after gel point (AGP) is introduced in the analysis of the mechanical properties during cure. A normalized elastic modulus is defined from the value at AGP, taken as a reference. The normalized elastic modulus is then compared to the polymerization degree. For pure resin samples, the logarithm of chemical conversion is found to be almost linearly related to the logarithm of the elastic modulus. Based on this comparison, a thermochemical model is proposed to describe the evolution of mechanical properties during the cure of composite samples with different fiber volume fractions. The viscoelastic behavior is also determined by performing stress relaxation tests with the DMTA. Resin specimens are tested for different cure states below the glass transition temperature, and master curves of stress relaxation during cure are constructed by applying the time-temperature superposition principle. The measurements depict the relaxation modulus of polyester resins as sharply affected by the degree of polymerization. Based on the experimental data, a relaxation modulus is modeled in a thermorheologically simple manner using exponential and power laws. Finally, a linear volume change model is constructed based on the TMA measurements of thermal expansion and resin shrinkage. The volume changes resulting from composite expansion-contraction and resin polymerization shrinkage are modeled as a function of temperature and degree of polymerization. The purpose of this work is to develop appropriate models of chemo- and thermomechanical behaviors of glass-polyester composites during cure. A resin cure kinetics model is developed by adding the glass transition effects to the J.L.B. model. For the mechanical properties, two new models are presented to account for the elastic and viscoelastic behaviors of the resin and the composite. Finally, the coefficients of the volume changes model are measured to account for the composite thermal expansion-contraction and resin chemical shrinkage. These models will be used in future investigations for thermal and curing optimization of composites processed by resin transfer molding.

Numerical analysis of cure temperature and internal stresses in thin and thick RTM parts

Abstract Resin transfer molding (RTM) is a widely used manufacturing technique of composite parts. Proper selection of processing parameters is critical in order to produce successful molding and to obtain a good part. Notably, when thermosetting resins are processed, the shrinkage that results from resin polymerization increases the complexity of the problem. Numerical prediction of internal stresses during composite manufacturing has three objectives: (1) to improve knowledge about the process; (2) to analyze the effects of processing parameters on the mechanical integrity of the part; and (3) to validate the principles of thermal optimization. This investigation aims to predict residual stresses and part deformation (i.e. warpage) in thin and thick composites. Accurate characterization of materials is essential for effective numerical analysis of phenomena which determine the generation of processing stresses. For this purpose, a reaction kinetics model of the resin is presented, together with a description of mechanical properties as a function of the degree of polymerization and glass transition temperature. A linear model is used to predict volume changes in glass–polyester composites. A finite difference analysis is used to simulate the effect of thermal and rheological changes during the processing of sample plates. Classical laminate theory is applied to calculate the internal stresses that result from processing conditions. These stresses are compared to determine different curing strategies for thick composite parts. Finally, a thermal optimization algorithm is applied to demonstrate the advantages of transient heating and cooling, to minimize processing stresses and avoid thermal degradation of the material or composite delamination.

Internal Stresses and Warpage of Thin Composite Parts Manufactured by RTM

Resin transfer moulding (RTM) is a widely used manufacturing technique of composite parts. A proper selection of process parameters is the key to yield successful moulding results and obtain a good part. Among other things, when thermoset resins are processed, the shrinkage that occurs due to the polymerisation reaction further complicates the situation. In this paper, a finite difference analysis is proposed to simulate the effect of thermal and rheological changes during thin plates cooling after processing. Classical Laminate Theory is here implemented to compute composite internal stresses resulting from these thermo-rheological conditions. Laminate stresses are then computed and warpage obtained with the proposed numerical algorithm. Samples of thin plates were moulded combining two glass reinforcement materials. During cooling, after processing plates warpage was recorded and results compared to model predictions. This analysis presents the basis of a further numerical optimisation for thick composite parts.

Fine grinding of silicon wafers

Silicon wafers are used for the production of most microchips. Various processes are needed to transfer a silicon crystal ingot into wafers. As one of such processes, surface grinding of silicon wafers has attracted attention among various investigators and a limited number of articles can be found in the literature. However, no published articles are available regarding fine grinding of silicon wafers. In this paper, the uniqueness and the special requirements of the silicon wafer fine grinding process are introduced first. Then some experimental results on the fine grinding of silicon wafers are presented and discussed. Tests on different grinding wheels demonstrate the importance of choosing the correct wheel and an illustration of the proper selection of process parameters is included. Also discussed are the effects of the nozzle position and the flow rate of the grinding coolant.

Structure and properties of decorative rare-earth hexaboride coatings

Hexaborides of the rare-earth elements are considered for applications as wear- and corrosion-resistant hard coatings for decoration of consumer products such as eye-glass frames and wristwatch casings. After a review of physical vapour deposition (PVD) methods used for the deposition of rare-earth hexaborides, a survey of studies is given that extends the range of suitable decorative coating materials. Within the scope of the search for new colours, the interdependence between deposition conditions, microstructure and mechanical and optical properties of boride coatings based on LaB 6, CeB 6, SmB 6 and YB 6 is presented and discussed. As a result of the strong directionally of covalent boron—boron bonds, hexaboride coatings show an increasing tendency to amorphous film growth with decreasing metallic radius of the rare-earth metal. Mechanical as well as optical properties are strongly influenced by the crystallographic structure of the hexaboride phase. The most pronounced visual colour impression is observed for LaB 6-based coatings corresponding to a dark violet colour. Proper selection of process parameters for these coatings may widen the conventional spectrum of decorative coatings.

Parametric studies of cutting zircaloy-2 sheets with a laser beam

The highly reactive and pyrophoric nature of zirconium alloys limits the use of conventional thermal sources (e.g., plasma arc cutting, oxygen flame cutting, etc.) for the cutting and drilling of these alloys. In this context, a highly coherent laser beam provides a good alternative for the cutting and drilling. In the present paper, laser beam cutting of zircaloy-2 sheets of 1.1 mm and 0.74 mm thickness is performed using a 300 W average power pulsed Nd:YAG laser. Pulse energy, pulse repetition rate, nozzle gap, gas pressure and cutting speed were varied to give different laser cutting conditions. Metallographic study of the cut surfaces showed the presence of transformed beta phase in the heat affected zone (HAZ) near the cut surface. The microhardness value across the cut surface was also measured. It showed a gradual increase in microhardness from the base metal (160 VHN) towards the HAZ having a maximum value of 365 VHN. The results of parametric studies of the cutting indicated that, with proper selection of process parameters, very narrow cuts can be easily made in zircaloy-2 using a pulsed Nd:YAG laser with a saving in material and at a much faster rate than alternative processes such as plasma arc cutting and oxygen flame cutting.

Decorative hard coatings: new layer systems without allergy risk

Combinations of physically vapour deposited (PVD) coatings and electroplated layers are widely used for decorative applications. The electroplated layer beneath the PVD coating acts as a diffusion barrier. Conventionally electroplated barrier layers make use of nickel which has to be replaced in allergy-free systems by for example Ni/NiPd and CuSn(Zn). The PVD coating has to meet a variety of requirements: attractive colour, high wear resistance and good corrosion resistance. Within the scope of the search for new colours a number of nitrides of binary IVa metal-based alloys such as (Ti,V)N, (Zr,V)N, (Zr,Al)N, (Zr,Cr)N, (Zr,Y)N were deposited by reactive magnetron sputtering. To describe the colour the CIE L ∗ a ∗ b ∗ system was used. Spectroscopic ellipsometry (SE) was introduced to determine material related parameters. In general, both the stoichiometry and the structure can influence the colour of a coating. The correlation between colour, stoichiometry and structure was determined by surface analytical methods (glow discharge optical spectroscopy, Auger electron spectroscopy, X-ray diffraction). Proper selection of process parameters (target composition, N 2 flow rate, etc.) results in coatings with colours exceeding the conventional spectrum of decorative coatings. As an example of structural and stoichiometric influences on colour, ZrN and (Zr,Y)N coatings are discussed.

Bias-induced stress transitions in sputtered TiN films

We report on intrinsic stress properties of magnetron sputtered titanium nitride films deposited under different conditions. By proper selection of processing parameters, films with low stress can be obtained. Unstressed film formation is favored by low substrate bias voltage, high pressure, or use of heavy sputtering gases. Stress relief is, however, accompanied by an increase in resistivity and a decrease in film density. As a result of these changes the effectiveness of such titanium nitride films as diffusion barriers between silicon and aluminum is minimal.

Contact resistance behavior of the Pd2 Si-AlCuSi system

Aluminum to silicon contact resistance, Rc , is influenced by the choice of intermediate contact and interconnection metallurgies, as well as anneal cycles associated with metallization and post-metallization passivation processes in single and multi-level metal VLSI technologies. This is particularly evident for the metallurgical system comprised of palladium silicide, Pd2 Si, and AlCuSi. This system has been investigated to ascertain the relationships between R , anneal time, and anneal temperature for various Pd Si thicknesses of AlCuSi contacts to N+ single-crystal silicon and polysilicon. The evaluations revealed that Rc and Rc distribution are inversely proportional to Pd2 Si thickness, and increase with anneal time and temperature. The results are compared to the known physical chemical interactions of Pd Si and Al. The studies demonstrate that by proper selection of process parameters and contact structure design, stable Al-Si contact resistance can be achieved for semiconductor device applications when Pd is used as an intermediate contact metallurgy.