Binders Used for the Manufacturing of Composite Materials by Liquid Composite Molding

Binders, or tackifiers, have become widespread in the production of new composites materials by liquid composite molding (LCM) techniques due to their ability to stabilize preforms during laying-up and impregnation, as well as to improve fracture toughness of the obtained composites, which is very important in aviation, automotive, ship manufacturing, etc. Furthermore, they can be used in modern methods of automatic laying of dry fibers into preforms, which significantly reduces the labor cost of the manufacturing process. In this article, we review the existing research from the 1960s of the 20th century to the present days in the field of creation and properties of binders used to bond various layers of preforms in the manufacturing of composite materials by LCM methods to summarize and synthesize knowledge on these issues. Different binders based on epoxy, polyester, and a number of other resins compatible with the corresponding polymer matrices are considered in the article. The influence of binders on the preforming process, various properties of obtained preforms, including compaction, stability, and permeability, as well as the main characteristics of composite materials obtained by various LCM methods and the advantages and disadvantages of this technology have been also highlighted.

Synthesis and Sintering of Nanostructured ZrB2-SiC Composite

ZrB2 is considered a candidate material for ultra-high temperature ceramics because of its high thermal conductivity, high melting point, and low coefficient of thermal expansion. Despite these attractive properties, applications of ZrB2 are limited by its low fracture toughness below the brittle-ductile transition temperature. To improve its ductile properties, the approach universally utilized has been to add a second material to form composites and fabricate nanostructured materials. One example of this is the adding of SiC to ZrB2 to improve fracture toughness. SiC has low density, excellent resistance to oxidation in air, and a high melting point. Therefore, SiC may be a promising additive as a reinforcing material for ZrB2-based composites. A dense nanostructured ZrB2-SiC composite was rapidly synthesized and sintered by high-frequency induction heating (HFIH) within 4 min in one step, from mechanically activated powders of ZrC, 2B and Si. Simultaneous combustion synthesis and consolidation were accomplished using the combination of current and mechanical pressure. A highly dense ZrB2-SiC composite with a relative density of up to 98.4% was fabricated using the simultaneous application of 70 MPa pressure and an induced current. The mechanical properties (toughness and hardness) and the average grain size of the composite were investigated.

Mode I fracture toughness of fiber-reinforced polymer composites: A review

Composite materials are known for their high stiffness and strength at lower weight as compared to conventional structural materials. Recently, there has been a growing interest in finding the new ways to decrease delamination failure, which is a life limiting factor of laminated composites. This review paper emphasizes on the effects of different reinforcement structures on mode I fracture toughness and possible ways to improve fracture toughness. A brief description on intrinsic and extrinsic mechanisms of crack growth has been discussed along with the earlier investigations and recent developments for mode I fracture toughness testing. Factors that affect the fracture toughness are also discussed. A brief knowledge of mode I fracture toughness of traditional and advanced fiber-reinforced composites is given, which could help researchers to understand fracture behaviors of composites and thus, it can help engineers to design composites with higher interlaminar strength.

Precipitation Hardening on Mechanical and Corrosion Properties of Extruded Mg10Gd Modified with Nd and La

To improve the mechanical and corrosion properties of Mg10Gd, Nd and La are added, and from that, the influence of precipitation hardening was studied. An increase in strength, by decreasing grain size and increasing the volume fraction of Rare Earth-rich precipitates, has been found when increasing the amount of alloying elements. Alloys containing La appear less ductile. Where crack propagation is studied using 3-point bending on Mg10Gd and Mg10Gd1Nd, the failure is mostly driven by twinning; the alloys with La show suppressed twinning, but crack initiation and propagation is caused by brittle and coarse precipitates. Precipitation hardening did not improve fracture toughness and was mostly based on strong grain growth and low solubility of La in Mg. With added alloying elements, the grain size was found to be slightly smaller in the T6 condition—precipitates seem to pin grain boundaries and therefore limit grain boundary mobility. Alloys containing Nd showed the best precipitation hardening response. Corrosion behavior, investigated by voltammetry and immersion, showed the best behavior in the precipitation-hardened condition. Corrosion rates and surface morphology are used to discuss corrosion properties.

In Situ Platelet Reinforcement of Alumina and Zirconia Matrix Nanocomposites – One Concept, Different Reinforcement Mechanisms

Zirconia-alumina composites are structural ceramics which due to their high strength and toughness are interesting in biomedical and engineering applications. Reinforcement of such materials with in situ formed platelets can improve fracture toughness and reliability, the mechanisms are however not yet fully understood. In this study alumina and zirconia based composites (ZTA and ATZ) reinforced with various hexaaluminates were investigated. In ZTA materials the main effect of platelets is the improvement of toughness as the the grain size distribution of the microstructure is broadened and transformability of the zirconia dispersion is improved. Crack deflection by platelets is unimportant, toughening is commonly achieved at the expense of strength and hardness. In case of zirconia based composites results are strongly depending on the type of stabilizer (Y-TZP or Ce-TZP) used and the type of hexaaluminates formed in situ. Here platelets can cause crack deflection and crack bridging. By variation of the composite recipes a multitude of compositions can be produced which have mechanical properties tailored for individual applications.

Microstructure and Hardness of Friction Stir Weld Bead on Steel Plate Using W-25%Re Pin Tool

One of the challenges that impede the use of the relatively new friction stir welding (FSW) process in joining steels and high temperature alloys, as well as dissimilar materials, is the development of the right pin tool material that can stand the severe welding conditions of these alloys. Recent developments in FSW tool materials include tungsten rhenium (W-Re) alloys. The ductile to brittle transition temperature of pure tungsten is reduced by the addition of rhenium (Re).. The addition of Re also improve fracture toughness of the alloy. The major focus of this paper is studying the process of making a friction stir welding bead on mild steel using a proprietary W-25%Re alloy pin tool and investigating the effects of process parameters (i.e. tool rotational and welding speeds) on microstructure, microhardness as well as tool reaction loads. Grain refining of the steel microstructure was observed in all beads. Certain process conditions produced a bead with needle like microstructure with the highest values of hardness. Reaction forces were found to increase with the increase in the tool welding speed and to decrease with the increase of the tool rotational speed. Although the spectroscopic analysis of the beads confirmed the diffusion wear of the tool, the overall tool has shown excellent resistance to mechanical wear.

Annealed Treatment Effect on the Mechanical Properties of the Cf/SiC Composites Using Y2O3/ Al2O3 Additions

In this investigation, the rare-earth oxide Y2O3combined with Al2O3served as sintering additives and commercial α-SiC powder were applied to fabricate Cf/SiC composites by hot-pressing sintering. The results proved that combination of Al2O3and Y2O3sintering additives was effective for densification of Cf/SiC composites. The influence of annealed temperature on the phase constitution, microstructure and mechanical properties of the Cf/SiC composites was detailed. The combination of grain bridging, crack deflection and fiber debonding can improve fracture toughness. Keywords: Pressing sintering, Mechanical Properties, Annealed Treatment

Development of Dense Hydroxyapatite Nanoceramic by Pressureless Sintering

In order to improve fracture toughness, nanocrystalline hydroxyapatite (HA) powder was used for fabricate dense HA nanoceramic. The nanocrystalline HA powder was obtained with ball milling for 12 h from calcined natural bovine bone. The green compacts of HA nanopowder were sintered by pressureless sintering with non-linear heating rate sintering at 1200°C for 3 h. Fracture toughness and hardness properties of HA nanoceramic have been assessed using indentation technique. The fracture toughness values of sintered nanosized-HA powder is 3.50.1 MPa.m1/2, which is within those of human cortical bone. The nanostructure with liquid phase sintering of sintered nanosized-HA powder contributed to the increase of fracture toughness. The toughening mechanisms of pull-out of nanocrystalline HA and grain boundary sliding have been observed.