20 January 2017, Volume 41 Issue 1
    

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    Technical Review
  • Research Status of TiO2 Based Varistor Ceramics
    WANG Yu, ZHANG Ke-min, LI Wen-ge, YAO Wei-hua
    Materials For Mechanical Engineering. 2017, 41(1): 1-6. https://doi.org/10.11973/jxgccl201701001
    Abstract ( ) Download PDF ( )   Knowledge map   Save
    The effects of some experiment parameters, such as powder size, sintering temperature, soaking time, the type of dopant and their content on electrical properties of TiO2 varistor ceramics are summarized, as well as their crystal microstructures. The electrical properties include varistor voltage, nonlinear coefficient, dielectric loss, dielectric constant and so on. It is pointed that the research focus is on the development tendency in miniaturization and multi-functionalization of TiO2 varistor ceramics.
  • Combined Technology and Application of Trivista Laser Confocal Raman Spectrometer in Ferroelectric Ceramic Research
    ZHANG Sa
    Materials For Mechanical Engineering. 2017, 41(1): 7-12. https://doi.org/10.11973/jxgccl201701002
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    In this paper, the superiority of Raman spectroscopy and the characteristics of confocal micro Raman spectroscopy are briefly introduced. According to the demands of scientific research and test in the field of materials science, the in-situ observation application cases for PLZT ferroelectric ceramic materials under the external fields action of polarization field, force electric field and temperature field with the combined technology of mechanical devices for Trivista laser confocal micro Raman spectrometer in recent years are summarized, the micro structure's changes and fatigue damage mechanisms of PLZT ferroelectric ceramic materials under external fields are analyzed and discussed by obtained Raman spectra.
    • Testing & Research
  • Effects of TiO2 Contents on Phase and Mechanical Properties of Plasma Sprayed Al2O3-TiO2 Coatings
    WENG Peng-xiang, GUO Ping, ZHENG Zhen-huan, YE Hui, LI Qiang
    Materials For Mechanical Engineering. 2017, 41(1): 13-19. https://doi.org/10.11973/jxgccl201701003
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    Four kinds of Al2O3-TiO2 coatings with different TiO2 contents were prepared on cast iron substrate by plasma spraying. X-ray diffraction (XRD) technique combined with the Rietveld whole pattern fitting method was used to determine the phase's content in different coatings. The effects of TiO2 content on mechanical properties of the coatings were investigated by means of the microhardness measurement and tensile testing. The results indicate that there are a large amount of γ-Al2O3, α-Al2O3 and a small quantity of amorphous phase in Al2O3-3wt%TiO2 coating; the maximum value of the amorphous content appears, and Al2TiO5 phase forms in Al2O3-13wt%TiO2 coating. With the further increase of the TiO2 content, the amount of the amorphous phase decreases, and Al2TiO5 phase's content increases in coatings. There are only α-Al2O3 and Al2TiO5 phases in the Al2O3-40wt%TiO2 coating. The microhardness of the coatings decrease but the adhesion strength enhance with the increase of TiO2 contents, this can be attributed to the formation of the Al2TiO5 phase.
  • Effect of Microstructures on Hydrogen Induced Cracking Behavior of Ultra-low Carbon X80 Pipeline Steel
    GE Qiu-chen, CHEN Jian, WANG Bing, LIU Qing-you, GAN Guo-you
    Materials For Mechanical Engineering. 2017, 41(1): 20-24. https://doi.org/10.11973/jxgccl201701004
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    Normalizing and quenching treatment were carried out on ultra-low carbon X80 pipeline steel, and then the influence of microstructures on the hydrogen-induced cracking behavior of the steel was investigated by optical microscopy, scanning electron microscopy, transmission electron microscopy and electron backscatter diffraction (EBSD) technology. The results show that the tested steel before heat treatment owned acicular ferrite structure, and its HIC resistance was better than the HIC resistance of polygonal ferrite structure after normalizing and bainite ferrite structure after quenching. The uniform distribution of high and low angle grain boundaries was of benefit to the improvement of HIC resistance of the test steel,meanwhile high density low-angle grain boundaries could prevent the extend of hydrogen induced crack.
  • Depth Extension Evaluation of Fatigue Cracks Initiated from Corrosion Pits on Aluminum Alloy 7A09 with Prior Corrosion Damage
    LI Xu-dong, LIU Yuan-hai, LIU Zhi-guo, ZHU Ze-long
    Materials For Mechanical Engineering. 2017, 41(1): 25-29. https://doi.org/10.11973/jxgccl201701005
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    The present fatigue life evaluation approach for aluminum alloy containing defects is based on depth extension of crack which is hidden inside and hard to be directly obtained. In order to find the relation between depth extension and surface cracks length, fatigue experiment was carried out on aluminum alloy 7A09 with prior corrosion damage, and propagation rules of corrosion pit cracks along depth direction was found by analyzing the fatigue fracture by using scanning electron microscope and energy disperse microscope. Results indicate that the fatigue fratography exhibits brittle fracture morphology, and the accumulation of the second phase particles will inversely enhance embrittlement by reducing plasticity. For fracture dominated by solo crack, the depth extension and the surface length of crack displays a linear relationship which provide a way to evaluate depth extension of a crack based on the detectable surface length, which is valuable to residual life prediction of structure. For multiple fatigue, there is uncertain relation between extension length and depth of crack.
  • Influences of Annealing Treatment on Microstructure and Mechanical Properties of TC18 Titanium Alloy Electron Beam Welded Joint
    ZHANG Si-cong, LEI Yong-ping, LIU Xin
    Materials For Mechanical Engineering. 2017, 41(1): 30-33. https://doi.org/10.11973/jxgccl201701006
    Abstract ( ) Download PDF ( )   Knowledge map   Save
    Firstly, normal annealing and stress relief annealing were carried out on electron beam welded joint of TC18 titanium alloys, and then the effects of annealing treatments on microstructure, tensile and impact properties of the welded joint were studied; the properties of the welded joint were compared with that of the base metal. It is shown that, compared with the stress relief annealing, the content of α phase in the normal annealed joint decreased significantly while the content of β phase increased dramatically. Compared with the stress relief annealing, the strength of the joint adopting normal annealing was lower, while its plasticity was higher. After normal and stress relief annealing treatments, the tensile properties of base metal and welded joint were similar, but the impact property of base metal was better than that of the welded joint.
  • Preparation of High-Performance Graphene/MnO2 Composite by Plant Biotemplate Method and Its Electrochemical Property
    HE Feng-juan, QIAN Jun-chao, LIU Cheng-bao, CHEN Zhi-gang, LI Ping
    Materials For Mechanical Engineering. 2017, 41(1): 34-37,42. https://doi.org/10.11973/jxgccl201701007
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    A graphene/MnO2 composite was prepared after dipping a plant membrane, which was used as both a carbon source and a template, into the manganese chloride solution and then calcining twice. The micromorphology, phase composition and surface property of the composite were analyzed. Then the composite was pressed to be an electrode and the electrochemical property was measured by cyclic voltammetry and constant current charge-discharge tests. The results show that there was no graphene oxide in the graphene/MnO2 composite. The graphene showed a lamellar structure with wrinkles on it, whose thickness was 1.449 nm; there were lots of MnO2 particles with the size of 10-40 nm and loaded on the graphene. The BET specific surface area of the composite was 202.5 m2·g-1 and the pore size was 5-30 nm. The composite had an excellent electrochemical reversibility with the specific capacitance of 245 F·g-1, which was much higher than that of commercial carbon powder (25-26 F·g-1) and synthesized graphene by the plant membrane calcination (45-50 F·g-1).
  • Microstructures and Mechanical Properties of Al-Zn-Mg Alloy after Adding Al-5Ti-B Refiner and Al-2Zr-Sc Intermediate Alloy
    DING Jian, DU Xi-wang, JIN Zi-hui, XIA Xing-chuan, ZHAO Nai-qin, ZHAO Wei-min, LIU Li-hua
    Materials For Mechanical Engineering. 2017, 41(1): 38-42. https://doi.org/10.11973/jxgccl201701008
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    Al-Zn-Mg alloy was refined with the single or compound addition of Al-2Zr-Sc intermediate alloy and Al-5Ti-B refiner. The microstructures and mechanical properties of as-cast Mg-Al-Zn alloys prepared under different refining conditions were studied. The results show that the grains were refined by single adding 0.4wt% Al-5Ti-B refiner or 0.4wt% Al-2Zr-Sc intermediate alloy, and the tensile strength was improved by Al3(Sc,Zr) phases after the addition of 0.4wt% Al-2Zr-Sc intermediate alloy. The refining effects of grains was best after compound addition of 0.2wt% Al-5Ti-B and 0.2wt% Al-2Zr-Sc, compared with the alloy before adding, the average grain size of the alloy after compound addition was reduced by 20 μm, and tensile strength was increased by 32%, which was result of grain strengthening and dispersion strengthening. The second phase with long rod shape caused the elongation of the alloy after compound refining was lower than that of the alloy after single refining.
  • Effects of Heat Treatment Processes on Corrosion Resistance of Welded Joint of FV520(B) Steels
    LI Ji-hong, LIU Juan-juan, ZHANG Min, XIE Wei-wei, FAN Qing-yang
    Materials For Mechanical Engineering. 2017, 41(1): 43-46,75. https://doi.org/10.11973/jxgccl201701009
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    Welding experiment was carried out on FV520(B) steels, and then different heat treatments were carried out on base metal and welded joint. Then corrosion experiments of base metal and welded joint were executed in 10wt% H2SO4 solution to study the effects of heat treatment processes on corrosion resistance of welded joint. The results show that, after different heat treatments, the surface corrosion morphology of FV520(B) steel welded joint was relatively uniform, but deep fault and ravined corrosion pits were found near the fusion zone. Under the same heat treatment conditions, the corrosion rate of base metal was higher than that of the welded joint. The corrosion resistance of welded joint and base metal could be improved through adding intermediate treatment between solid solution and aging processes. After intermediate treatment at 850℃, the welded joint owned more uniform surface morphology than the welded joint after intermediate treatment at 750℃.
  • Phase and Properties of Mo Alloyed Layer on TiNi Alloy Surface
    SHAN Xiao-lin, WU Hai-rui, WANG Zhen-xia, LIN Nai-ming, LIU Xiao-ping, HE Zhi-yong
    Materials For Mechanical Engineering. 2017, 41(1): 47-50,55. https://doi.org/10.11973/jxgccl201701010
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    Mo alloyed layer on the surface of TiNi alloy substrate was obtained by using the double glow plasma surface alloying technique, and phase and composition distribution of the Mo alloyed layer were investigated. The electrochemical performance and wear resistance of Mo alloyed layer and TiNi alloy substrate samples were studied by by ultrasonic vibration cavitation erosion test machine with electrochemical measurement system and the reciprocating friction and wear tester. The results show that the atomic concentration of Mo alloyed layer presented gradient distribution and Mo alloyed layer was mainly composed of Mo, MoTi and Ni3Ti phases. The passivation capability of the substrate was suppressed during the cavitation, while Mo alloyed sample showed the passivation in a certain extent. The wear mechanism of Mo alloyed sample and the substrate alloy sample were abrasive wear and adhesive wear. The wear scar width of TiNi alloy substrate was much larger than that of Mo alloyed sample, and the adhesive wear of the substrate alloy was more serious.
  • Effect of Vertical Load on Fretting Damage Characteristic of Shaft Surface in Torsion
    WEN Guang, HE Cheng-gang, ZHOU Gui-yuan, LIU Qi-yue
    Materials For Mechanical Engineering. 2017, 41(1): 51-55. https://doi.org/10.11973/jxgccl201701011
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    Fretting damage on shaft surface in torsion was studied by using JD-1 wheel/rail simulation tester, and the effect of vertical load on fretting damage characteristic of interface fit between shaft and inner surface of bearings was investigated. The fretting damage mechanism of shaft surface was analyzed by observing the surface wear scar and microstructure of cross section of specimens by using laser scanning confocal microscopy and scanning electronic microscopy. The results indicate that the fretting damage appeares on the interface fit between shaft and inner surface of bearings at vertical loads, the wear is mainly in the form of adhesive wear and abrasive wear. Under higher vertical load condition, the wear on the surfaces of shaft is more serious, plastic layers in the interface fit both become thicker, the fretting damage becomes severer, moreover, the wear and plastic deformation on the left surface are severer than those on the right surface, and the thickness of the plastic layer is uneven.
  • Effects of Pulsed Negative Bias Amplitudes on Structure and Properties of TiCN Film Prepared by Compound Ion Plating Technology
    REN Xin, ZHAO Rui-shan, HUANG Mei-dong, ZHANG Chong-yi, WANG Qian-bao
    Materials For Mechanical Engineering. 2017, 41(1): 56-60,64. https://doi.org/10.11973/jxgccl201701012
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    TiCN film was prepared on 304 stainless steel at different pulse negative bias amplitudes by the compound ion plating technology. The effects of pulse negative bias amplitude on composition, structure, surface roughnesss, microhardness and friction and wear properties of the film were studied. The results show that, with the increase of negative bias amplitude, the atom ratio of Ti and C to N first increased and then decreased, as well as the ratio of C to Ti. The size of particles, pinholes and pits on the film surface decreased, and their quantity also reduced with the increase of negative bias amplitude, these improve the surface morphology of the film. The film was mainly composed of TiCN phase, presenting the (111) preferred orientation with the increase of negative bias amplitude. In addition, microhardness of the film first increased then decreased with the increase of negative bias amplitude, reaching a maximum of 2 690 HV at 300 V. Friction and wear properties of the film are always better than that of the substrate, and the friction coefficient of TiCN film decreased with the increase of negative bias amplitude; when the negative bias amplitude was 300 V, the friction coefficient was up to 0.355, indicating that the optimal friction and wear properties was achieved.
  • Interface Structure and Bonding Properties of Cu/AlN Material Fabricated by Direct Bonded Copper Process
    XIE Jian-jun, WANG Yu, WANG Tun, WANG Ya-li, DING Mao-mao, LI De-shan, ZHAI Tian-lei, LIN De-bao, ZHANG Lei, WU Zhi-hao, SHI Ying
    Materials For Mechanical Engineering. 2017, 41(1): 61-64. https://doi.org/10.11973/jxgccl201701013
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    By direct bonded copper (DBC) process, Cu/AlN material was fabricated successfully on AlN ceramic substrate at bonding temperatures of 1 000-1 060℃. The bonding strength, morphology and phase composition of Cu/AlN interface were investigated by mechanic testing equipment, field emission scanning electron microscope and X-ray diffractometer. The results show that the bonding strength of Cu/AlN interface was more than 8.0 N·mm-1, a transition layer with thickness of 2 μm was found between copper foil and AlN ceramic, the transition layer was mainly composed of Al2O3, CuAlO2 and Cu2O. The bonding strength of Cu/AlN interface increased as the bonding temperature gradually went up.
  • Microstructures and Mechanical Properties of A356 Aluminum Alloys after Modificated by Adding Amorphous Alloy
    SU Yun-gao, CUI Zhen-duo, ZHU Sheng-li, ZHAO Nai-qin, DING Jian, ZHAO Wei-min
    Materials For Mechanical Engineering. 2017, 41(1): 65-71. https://doi.org/10.11973/jxgccl201701014
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    A356 aluminum alloy with Al-5Ti-1B pre-refine treatment was modified by adding various amounts of Zr56Co22Al16Ag6 amorphous alloy ribbons, and then solid solution and aging heat treatment was executed on the aluminum alloy. The microstructure and mechanical properties of the aluminum alloy were studied, and the amorphous ribbon's refine mechanism was also discussed. The results show that with the increase of amorphous alloy ribbon's adding amounts, grain size and the secondary dendrite spacing of the aluminum alloy all decrease, and the mechanical properties of the aluminum alloy are improved. Dispersed granular Al3Zr phase is found in eutectic structure, the phase increases the number of nucleus, hence refines the grains. Adding 0.4wt% Zr56Co22Al16Ag6 amorphous alloy ribbons and setting incubation period of 10 min could make the aluminum alloy obtain the best refining effect, and after solid solution and aging treatment, tensile strength and elongation of the aluminum alloy owned the higher mechanical properties, compared with the aluminum alloy without adding amorphous ribbons, its tensile strength and elongation increased by 7%and 64.4%, respectively.
    • New Materials & Technology
  • Research and Development of Slab Continuous Casting Sulfur Y55 Steel Plate and Its Properties
    DONG Fu-jun, ZHAO He-ming, LIU Xiao-lin, YANG Cai-fu
    Materials For Mechanical Engineering. 2017, 41(1): 72-75. https://doi.org/10.11973/jxgccl201701015
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    In order to meet manching and usability needs of mold steel, a slab continuous casting sulfur Y55 steel plate was successfully developed through adding free-cutting element sulfur, appropriate manganese and calcium, and optimizing smelting, continuous casting and rolling processes; its mechanical and cutting performances were analyzed.The results show that the hardness of Y55 steel plate was 180-200 HB, the steel's microstrucure was pearlite and reticular ferrite, and the inclusions were composite inclusion mainly MnS-oriented. Compared with S50C steel plate, the machining performance of Y55 steel plate was improved sharply.
  • Effects of Cr3C2 Nanoparticle Content on Microstructure and Property of Plasma Surfacing Co40 Alloy Coating
    SI Song-hua, YU Wan-ping, YAO Chang-bin, ZHANG Lei
    Materials For Mechanical Engineering. 2017, 41(1): 76-79. https://doi.org/10.11973/jxgccl201701016
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    The Co40 alloy coatings with different Cr3C2 nanoparticle content (0, 20wt%, 30wt%, 40wt%) were obtained by plasma surfacing on low-carbon steel Q890, and then the effects of the Cr3C2 nanoparticle content on the microstructure and wear resistance of the alloy coating were studied. The results show that the Co40 alloy coating was composed of developed columnar crystals and netlike eutectic crystals among them. With the addition of Cr3C2 nanoparticle, the microstructure was refined and the solidification characteristic changed from the hypoeutectic crystallization to the hypereutectic crystallization. Moreover, the hardness and wear resistance of the alloy coating were significantly improved. When the Cr3C2 nanoparticle addition was 30wt%, the wear mass loss of the alloy coating was the lowest, which decreased by 55.8% comparing to that without Cr3C2; the wear surface was relatively smooth with only little slight furrows and almost no peeling phenomenon was observed.
    • Material Properties & Application
  • Wear Resistance of Low Alloy High Strength Steel with Different Microstructures and Same Strength Grade
    SI Song-hua, TANG Huai-guang, HU Xue-wen, ZHONG Yao-shun
    Materials For Mechanical Engineering. 2017, 41(1): 80-84. https://doi.org/10.11973/jxgccl201701017
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    Using the friction and wear testing machine and free abrasive wear testing machine, the wear resistance and wear mechanism of the low alloy high strength steels S700 and M700 with two different microstructures but same tensile strength level of 700 MPa were studied under different lubrication, loading and corrosion conditions; their wear mechanism were analyzed. The results show that the microstructure of S700 steel is ferrite and bainite, and the microstructure of M700 steel is ferrite and pearlite, and the hardness of S700 steel is slightly higher than that of M700 steel. Under the same sliding wear condition, the wear mass of S700 steel is less than that of M700 steel. The wear resistance of test steels under acidic condition is poor. Under the same corrosion wear conditions, the corrosion resistance of S700 steel is better than that of M700 steel. The sliding wear surface morphology of the steels is mainly micro cutting mechanism and plastic deformation mechanism; their corrosion wear morphology in acidic solution appears large area of corrosion pits and accompanied by spalling, and the morphology in alkaline solution is micro cutting, accompanied by a little peeling.
  • Properties of Resin-based Composite Friction Materials with Different Contents of Modified Bauxite
    LU Shang-jian, FU Xue-song, SUN Wei-tao, ZHOU Wen-long, CHEN Guo-qing
    Materials For Mechanical Engineering. 2017, 41(1): 85-89. https://doi.org/10.11973/jxgccl201701018
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    By adding modified bauxite by 4%-12% (mass fraction) and reducing the addition of phenolic resin, resin-based composite friction materials were prepared, and the mechanical properties, anti-fade properties and friction and wear properties of the resin-based composite friction materials were studied. Experimental results show that, after adding 8%(mass fraction) modified bauxite, hardness, flexural strength and impact toughness of the composite decreased slightly, but still in the high level, and friction coefficient increased considerably. The high temperature anti-fade coefficient of the composite friction materials increased by nearly 12% than the material without modified bauxite, and they exhibited better anti-fade properties.
    • Physical Simulation & Numerical Simulation
  • Prediction and Optimization of Mechanical Properties of Multi-phase Anisotropic TiB2 Reinforced Steel Composite Using Model
    LI Han, JIN Xue-jun, ZHAO Hong-shan
    Materials For Mechanical Engineering. 2017, 41(1): 90-95,106. https://doi.org/10.11973/jxgccl201701019
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    An analytical computational model was built based on the Mori-Tanaka method and Eshelby's equivalent inclusion principle, which was used to predict mechanical properties of multi-phase anisotropic TiB2 reinforced steel composite, and the effects of reinforcement's aspect ratio, volume fraction and orientation on longitudinal and transverse elasticity modulus of the composite were studied. Results show that the analytical model can predict different kinds of metal-matrix composites with high level precision. The reinforcements with larger aspect ratio can significantly raise the composite's longitudinal modulus, while diminishing the transverse modulus only by a small amount. The volume fraction of reinforcement always has a positive correlation with elasticity modulus. With the increase of the angle between the alignment of the reinforcement and the loading direction, the longitudinal modulus increases first and then decreases.
  • Finite Element Modelling for Bending Stress of Polycrystalline Silicon Wafer with Different Grain Sizes and Crystal Orientation Distributions
    WANG Yi-lun, DING Jian-ning, YUAN Ning-yi, JIANG Cun-hua, CHEN Xiao
    Materials For Mechanical Engineering. 2017, 41(1): 96-102,106. https://doi.org/10.11973/jxgccl201701020
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    The photoluminescence (PL) technology was used to identify the crystal orientation of the polycrystalline silicon wafer, the elastic modulus of polycrystalline silicon wafer in different orientations was measured through the nano-indentation experiment. The finite element program was used to establish the polycrystalline silicon wafer finite element model containing the information of grain size and grain orientation, and then the elastic modulus got from the nano-indentation experiment were inputted to the model to obtain the bending strength of polycrystalline silicon wafer with different grain sizes and grain orientation distributions; the simulation results were verified by three-point bending experiment. The results show that elastic modulus and hardness of polycrystalline silicon wafter in different grain orientation distributions are different. The maximum bending stress value and the maximum stress position of the polycrystalline silicon wafer are affected by crystal orientation distribution. Grain shapes will affect the maximum bending stress. Reducing the grain size can reduce the maximum bending stress.The model's correction is verified by the three-point bending experiment.
  • Hot Compression Deformation Behavior of 20CrNi2Mo Steel at Different Deformations
    FAN Hang-jing, LIANG Yi-long, LEI Lei, LONG Shao-lei, YANG Ming
    Materials For Mechanical Engineering. 2017, 41(1): 103-106. https://doi.org/10.11973/jxgccl201701021
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    Thermal simulation compression experiment was carried out on 20CrNi2Mo steel by using phase change instrument, and the steel's thermal compressive deformation behavior at 840℃ under different deformations (0-50%) was studied. The steel's microstructure and hardness were also analyzed. The results show that the flow stress curve of 20CrNi2Mo steel exhibits obvious dynamic recrystallization characteristics. When the deformation amount reaches 10%, the dynamic recrystallization occurs. With the increase of the thermal compression deformation, the grain size is refined and subsequently can almost be kept stable. When the deformation amount reaches 50%, the specimen is completely recrystallized. In addition, with the increase of the thermal compression deformation, martensite is refined gradually, the content of residual austenite decreases. When the compression deformation changes from 10% to 20%, the hardness increases significantly, and then tends to stabilization.
  • Microstructure Transformation Characteristic of CGHAZ of Q1100 High-Strength Steel at Different Cooling Rates
    FANG Jun-fei, XU Zhen-lin, SI Song-hua, XIANG Xiao-cao
    Materials For Mechanical Engineering. 2017, 41(1): 107-110. https://doi.org/10.11973/jxgccl201701022
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    The welding process of Q1100 high-strength steel was simulated with Gleeble-3500 thermal simulator, and then SHCCT(simulated HAZ continuous cooling transformation) diagram of the steel was obtained by using thermal expansion measurement combined with microstructure analysis and hardness testing. The microstructure transformation characteristics and hardness change rule of coarse-grained heat-affected zone (CGHAZ) were studied at different cooling rates. The results show that, the austenitizing temperature of the steel in simulated welding condition was significantly higher than that in the equilibrium state. The CGHAZ was full bainite when the cooling rate was under 2℃·s-1, a mixed microstructure of bainite and martensite was obtained when the cooling rate was 2-12℃·s-1, and the CGHAZ was full martensite when the cooling rate was over 12℃·s-1. The hardness of CGHAZ increased gradually with the increase of cooling rate.
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