The running-in and steady-state coefficient of friction of some engineering thermoplastics

The dry friction properties of three engineering thermoplastics, ultra high-molecular-weight polyethylene (UHMWPE), polyoxymethylene (POW) and polyamide 66 (PA 66), have been investigated using a plane on plane contact. During running-in, the steady-state of the friction coincides with stable values for the surface temperature of the slider. The proper trend as well as the correct order of magnitude are observed between the measured surface temperatures and those calculated using the equation of Lancaster. The increase of the run-in friction as function of the P_v factor could be well correlated by μ(t) = μ_ss ? Pexp(? 2 10⁴ t), where μ_ss is the steady-state value of the coefficient of friction, n is a constant depending on the material tested. A decrease of μ_ss with increasing load for UHMWPE and POM indicates that friction is primarily the result of adhesion. However, for the PA 66, the opposite is valid, assuming that in this case the high T_g of this material may play an important role. Friction increases as the rugosity of specimen diminishes and as speed increases. Finally, the coefficient of friction could be well correlated by μ = kP^mexp(βv) where k, m, and β are constants depending on roughness and type of material.     

Tribological properties of DLC films deposited on steel substrate with various surface roughness

Tribological properties of diamond-like carbon (DLC) films in water were investigated concerning with the influence of surface roughness and various mating materials. The DLC films were deposited by pulsed-bias CVD method on AISI630 stainless steel. The substrate roughness (R_a) is in the range of 1.4–740 nm. AISI 440C, AISI 304 stainless steel and brass balls were used as a mating ball. The friction coefficients of DLC films against with AISI 440C stainless steel ball indicated under 0.1 irrespective of the roughness. The film having smooth surface (R_a=1.4 nm) had severe damage at a load of 9.4 N. However, the film having rough surface (R_a=263 nm) had no damage at the same load. The specific wear rate of the steel ball increased with increase of roughness of the surface. In the case of AISI 304 stainless steel ball, the specific wear rate of the ball showed similar tendency. The friction with brass ball showed relatively high friction coefficient in the range of 0.12–0.25. However, the damage on the films could not be observed after friction test. It is considered that the roughness of the surface is important factor for the rupture of the film in water environment.     

The viscosity of concentrated polymer solutions containing low molecular weight solvents

The zero shear rate viscosities of polystyrene/ethylbenzene solutions having polymer weight fractions ranging from 0.5 to 1.0 have been measured using a novel sealed rheometer cell over a temperature range of 50 to 200°C. The concentration and temperature dependence of the solution viscosity has been found to be well described by the relation η₀ = K c^aM_w ^3.4ζ(c, T) where the monomeric friction coefficient ζ is determined by the free volume of the solution. Following the procedure of Berry, the free volume parameters, α_f(c)/γ and T_∞ (c), and the fractional free volume, f(c,T)/γ, have been determined. After using these parameters to account for the concentration dependence of the friction coefficient, the concentration exponent a has been evaluated and found to be in reasonable agreement with the value of 3.4 obtained by Berry and Fox for other polymer/solvent systems. A comparison of the relative conributions made by the friction coefficient and the term c^3.4 to the overall concentration dependence of the viscosity of these highly concentrated solutions shows the friction coefficient to be the dominant factor     

A new slurry infiltration method to enhance the wear resistance of bulk graphite with development of reinforced graphitic composites including SiC or Si3N4 hard particles

Bulk graphite blocks are infiltrated by a Si slurry method to form composites of graphite containing SiC or Si₃N₄ reinforcements, in order to enhance the wear resistance of the graphitic structure. The microstructure of the SiC reinforcements includes nuclei grains and whiskers, while the microstructure of the Si₃N₄ reinforcements is a mixture of fine grains, grains of blade- and needle-like morphology. The wear rate of the SiC- and Si₃N₄-reinforced graphitic block is 77.7 and 42.8 μm³/Nmm, respectively, as measured using an unlubricated pin-on-disc test. These values are ?55% and 75% lower than the wear rate of the reference graphite (174 μm³/Nmm). The coefficient of friction of the composites is as low as the coefficient of friction of the reinforcement-free graphite, showing values of ?0.17.     

Young's modulus and adhesion coefficient of amorphous Ni–P coatings on a metal substrate

In the present paper the Young's modulus and adhesion coefficient of amorphous Ni–P coatings obtained from aqueous solutions were determined. The measurements were carried out using a vibrating reed apparatus. In the temperature range 550–590 K, crystallization of Ni and formation of nickel phosphide Ni₃P were observed. The Young's modulus of Ni–P amorphous layers on stainless steel at room temperature was found to be about 112 GPa. The adhesion coefficient γ of the examined layers depends on the layer thickness a _f and strongly decreases for a _f > 8 μm. This dependence corresponds to the change of the relative adhesion coefficient of about 40% for 8 μm < a _f < 15 μm. It was also shown that the adhesion coefficient does not depend on the temperature, at least in the range 300–550 K.     

Fabrication of high-purity HfSi2 powder via molten salt-assisted magnesium thermal reduction

In this work, high-purity HfSi₂ powders were successfully fabricated via a molten salt-assisted magnesium thermal reduction method using HfO₂ and Si as raw materials. The effects of reaction temperature and time on the formation of HfSi₂ were systemically investigated. The morphological and phase composition of as-prepared HfSi₂ powders were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and nitrogen/oxygen determinator. The results indicated that as-obtained HfSi₂ powders possess the orthorhombic structure with approximately 6.4 μm in size and the oxygen content as low as 0.20%. This work can provide a novel route to fabricate the high-purity transition metal silicides powders.     

Tribological behavior of ceramic materials (Si3N4, SiC and Al2O3) in aqueous medium

The friction and wear behavior of self-mated Si₃N₄, SiC and Al₂O₃ in water were investigated by varying the test conditions of applied load and sliding speed. It was found that, for self-mated Si₃N₄ and SiC ceramics, the tribochemical reaction resulted in surface smoothening with low friction coefficient at high load and high speed condition. Al₂O₃ shows high friction coefficient, but better wear rate (10⁻¹¹ mm²/N) than other ceramic materials.     

Selective removal of magnesium from lithium-rich brine for lithium purification by synergic solvent extraction using β-diketones and Cyanex 923

In the production of battery-grade and high-purity Li₂CO₃, it is essential to remove magnesium impurities. The state-of-the-art solvent extraction (SX) process using Versatic Acid 10 and D2EHPA co-extracts 3.3–5.5% lithium, while removing 86–98% magnesium. Here, we demonstrate that synergic SX systems containing a β-diketone (HPMBP, HTTA or HDBM) and Cyanex 923 are highly selective for magnesium extraction over lithium (separation factor α > 1,000). The extracted magnesium and lithium complexes have the stoichiometry of [Mg∙A₂∙(C923)₂] and [Li∙A _x∙(C923)₂] (x = 1, 2), respectively (A represents deprotonated β-diketone). The three β-diketone synergic SX systems all considerably outperformed the Versatic Acid 10 system for magnesium removal from a synthetic solution containing 24 g L⁻¹ Li and 0.24 g L⁻¹ Mg. In a three-stage batch counter-current extraction, the HPMBP and Cyanex 923 synergic SX system removed 100% magnesium with only 0.6% co-extraction of lithium. This excellent Mg/Li separation is the best result reported so far.     

Effects of Different Metal Fuels on the Characteristics for HTPB‐based Fuel Rich Solid Propellants

The microstructures and granularity distribution of different metal particles were investigated and the energy, sensitivity, and combustion properties of fuel rich solid propellants with different metal particles were studied in detail. It was found that the magnesium particles are more uniform than other metal powders, the mean diameter of the magnesium particles d₅₀=67.6 μm is much higher than those of the other ones, which are in the range of 7.1 μm<d₅₀<20.5 μm. Additionally, the preparation process of the Mg‐based propellant is easier than those of the other ones. The experimental results showed that the propellant containing magnesium powder was less sensitive to friction and impact (165.1 NM and 21.9 NM, respectively), whereas, the burning rates of propellants with Zr and ZrH₂ particles increased, and the pressure exponents decreased.     

Solvent effects on mutual diffusion in concentrated polystyrene solutions

In a previous paper, mutual diffusion coefficients for concentrated solutions of a monodisperse polystyrene in 14 solvents were reported. In this paper, these data are reinterpreted in terms of new thermodynamic data for these solvents. These new results permit evaluation of the activity derivative, ? In a₁/? In v₁, and therefore enable calculation of the diffusional friction coefficient from the mutual diffusion coefficient. The friction coefficient was found to be directly proportional to solvent viscosity to within the experimental errors involved in the combined transport and thermodynamic measurements. The molecular implications of this result are discussed.     

Effect of load on the friction and wear characteristics of Si3N4-hBN ceramic composites sliding against steels

The tribological behaviors of Si₃N₄-hBN ceramic composites sliding against steels (austenitic stainless steel (ASS) and 45 steel) under dry friction conditions at different loads were investigated by using an MMW-1 type vertical universal friction and wear tester. The experimental results showed that the friction coefficients and wear rates first showed a decrease and then an increase with an increase in the load under dry friction conditions. The better tribological performance was exhibited by the SN10/ASS sliding pair under a load of 20 N (the friction coefficient was as low as 0.27 and the wear rates of both pin and disc had a magnitude of 10⁻⁶ mm³ N⁻¹ m⁻¹). This may be attributed to the formation of a black surface film (consisting of B₂O₃, SiO₂, and Fe₂O₃). For the same sliding pair, when the load was 10 N, the dominating wear mechanism was abrasive wear. Hence, the friction coefficient was higher (0.7). When the load increased to 30 and 50 N, the wear mechanism of the SN10/ASS sliding pair was a combination of abrasive and adhesive wears, and higher friction coefficients (0.48 and 0.72 under loads of 30 and 50 N, respectively) were obtained. On the other hand, the contents of hBN also showed a significant impact on the tribological behaviors of the Si₃N₄-hBN/ASS sliding pairs. When the hBN content was less than 10%, the friction coefficients of the Si₃N₄-hBN/ASS sliding pairs decreased with an increase in the hBN content. On the other hand, at hBN contents of 10% or more, the friction coefficients of the sliding pairs increased with an increase in the hBN content. Under the same experimental conditions, the Si₃N₄-hBN/45 steel pairs showed poor tribological properties as compared with the Si₃N₄-hBN/ASS pairs.     

Influence of oxygen and medium conditions on the performance of a tower reactor with an external loop

Hansenula polymorpha was cultivated using ethanol and/or glucose as substrate in a bubble column, air lift, loop reactor. The optimum aeration rate was determined for extended culture operation. Considerable influence of the ethanol concentration on the volumetric mass transfer coefficient (k_La), and the specific gas-liquid inter-facial area (a) was found. Glucose concentration had no comparable influence on k_La and a. The influence of lack of substrate and/or dissolved oxygen on the culture was investigated. For 30 min the absence of substrate did not influence μm, whereas a lack of dissolved oxygen for 2.5 min reduced μm to one half of its original value.     

Nanocrystalline diamond coatings on the interior of WC–Co dies for drawing carbon steel tubes: Enhancement of tube properties

Tungsten carbide (WC–Co) dies are commercially used for the tube drawing process. However they wear out progressively and are unable to meet the high demands required by the industry. In this study, the effect of nanocrystalline diamond (NCD) coatings on the interior of WC–Co drawing dies using a hot filament chemical vapour deposition technique is reported. A field trial was conducted on the production line for drawing AISI 1541 steel tubes to investigate the quality of the drawn tubes. The surface roughness of the tubes drawn through the NCD coated die was lower (R_a = 381 nm) when compared to the tubes drawn through a regular carbide die (R_a = 527 nm). The average residual stress of tubes drawn through the NCD coated drawing die was lowered by 25%. A pin-on-disc sliding wear test, carried out to estimate the coefficient of friction, showed that the coefficient of friction in the case of the NCD coated die was almost half that of the regular WC–Co dies. The excellent thermal conductivity and lower friction coefficient of NCD coatings also helped to decrease the working temperature of the tube drawing process, thereby resulting in a superior product.     

Tribological Characteristics of α-Alumina at Elevated Temperatures

The tribological characteristics of a high-purity α-alumina sliding on a similar material under unlubricated conditions are divided into four distinct regimes. At low temperatures, T < 200°C, tribochemical reactions between the alumina surface and water vapor in the environment control the tribological performance. The coefficient of friction in this temperature range is approximately 0.40 and the wear coefficient is less than 10⁻⁶, independent of contact load. At intermediate temperatures, 200°C < T < 800°C, the wear behavior depends on the contact load. At low loads, wear occurs by plastic flow and plowing; the coefficient of friction is approximately 0.60 and the wear coefficient is less than 10⁻⁶. At loads larger than a threshold value, severe wear occurs by intergranular fracture. The coefficient of friction increases to 0.85 and the wear coefficient increases to a value greater than 10⁻⁴. At temperatures above 800°C, formation of a silicon-rich layer on the wear track by diffusion and viscous flow of the grain-boundary phase reduces the coefficient of friction to 0.40, and the wear coefficient is reduced to a value less than 10⁻⁶. The results of the wear tests and observations of the fundamental mechanisms controlling the tribological behavior of this material are consolidated in a simple wear transition diagram.     

Tribological properties of NCD coated cemented carbides in contact with wood

Cemented carbides coated with diamond layers are promising materials for mills in the wood industry. Therefore, a study of the tribological properties of the contact between this material and wood is interesting and important. Wood is a specific material with a highly anisotropic structure, which causes roughness of its surface. For example, the friction coefficient (μ) of wood in contact with polished steel coated with a smooth DLC layer, has a relatively high value of μ=0.2–0.5. Cemented carbides, as manufactured for tools for wood milling purposes with roughness R_z=0.66 μm, have been modified with a nanocrystalline diamond film (NCD) using the RF PACVD method. The surface of the NCD coating showed ‘sharp hills’ morphology, but the surface roughness of cemented carbide decreased slightly after coating. The friction was very high (μ=0.7–0.8 at v=1 m/s; F_N=60 N) and it depended on the species of wood. When examining oak and poplar using carbides coated with the thickest NCD layer and a DLC film on top, this value decreased by 30% with respect to that of uncoated carbides. For fibreboard against NCD, the value was increased. Comparing the friction of NCD against wood to its friction against steel, titanium and aluminium alloys, we could see that the roughness of wood was the main factor which determined its tribological behaviour. Prime novelty: the tribological properties of a NCD layer deposited on cemented carbides with wood.     

Mass transfer characteristics in a gas-liquid reciprocating plate column. I. Liquid phase volumetric mass transfer coefficient

In a series of two papers the results of investigating the mass transfer characteristics of two gas-liquid reciprocating plate columns of the Karr type by different methods are presented. The subject of the first part is a study of the liquid phase volumetric mass transfer coefficient, k_La, while the second part deals with the interfacial area. The volumetric coefficient k_La was investigated using the sulphite method, the pure physical absorption of oxygen, and a dynamic method under culture conditions, the second of these three methods being the most favorable. Very good agreement among these methods was found. Generally, k_La increased with increasing vibration intensity, superficial gas velocity, and the number of perforated plates. Liquid-phase properties appeared to affect k_La only slightly. The coefficient k_La was correlated in terms of the maximum power consumption and the superficial gas velocity:      

Tribological properties of partly polished diamond coatings

Extremely low friction coefficient was achieved with “partly polished diamond coatings”. Diamond coatings were deposited onto Si substrates by MWCVD with the mixture of CH₄ and H₂. Deposited films were characterized by X-ray diffraction (XRD), Raman spectroscopy and Electron Spectroscopy for Chemical Analysis (ESCA). Sharp peak derived from polycrystalline diamond was observed by XRD. Whereas Raman profile of partly polished diamond coatings was close to that of ta-C. This result suggests that small diamond grains were surrounded by amorphous carbon structure in the diamond coatings. Deposited diamond coating was polished with each other. Surface roughness R_a was reduced to 0.3, 0.2 and 0.08 μm, respectively. The hardness of the polished diamond coatings investigated by Nanoindentation technique was approximately 40.8 GPa, which was relatively lower value compared with conventional as-deposited CVD diamond coatings. For the tribological properties, we examined the effect of surface roughness using flat-ended pin-on-disk apparatus and ball-on-disk apparatus with bearing ball (SUJ2) and stainless steel (SUS304). Diamond coatings were deposited onto flat-ended pin and disk, and they were polished to R_a = 0.3, 0.2 and 0.08 μm. After the 6000 cycle process extremely low friction coefficient, μ = 0.05, was achieved with the pair of R_a (flat-ended pin, disk) = R_a (0.08, 0.3) in flat-ended pin-on-disk apparatus. In order to clarify the effect of surface roughness, ball-on-disk was carried out with different surface roughness, R_a = 1.7, 0.3, 0.2 and 0.08 μm. Here as-deposited diamond coating, R_a = 1.7 μm, was used as a reference point. Friction coefficient of μ = 0.09 was obtained for both balls. After the tribological tests balls were analyzed by scanning electron microscope (SEM) and energy dispersed X-ray spectrometer (EDX).     

Frictional behavior and wear resistance performance of gradient cermet composite tool materials sliding against hard materials

Gradient cermet composites possessing high surface hardness, flexural strength and interface bonding strength were fabricated using vacuum hot-pressing sintering. Ball-on-disk tests were performed to investigate the tribological properties of the gradient cermet composites against 440 C stainless steel, Al₂O₃ and Si₃N₄ balls at different sliding speed and load in comparison with traditional Ti(C,N) cermets. The tribological behavior was characterized in terms of friction coefficient and wear rate. The results showed that friction coefficient was significantly dependent on the sliding speed and load when sliding against Al₂O₃ and Si₃N₄. However, there was no obvious relation between them during sliding against 440 C stainless steel due to the formation of metal adhesive layer. Gradient cermet composites exhibited a higher friction coefficient but lower wear rate than traditional Ti(C,N) cermets. The main wear mechanism of gradient cermet composites was adhesion wear during sliding against 440 C stainless steel, while abrasion wear was the predominant mechanism during sliding against Al₂O₃ and Si₃N₄. It was expected that gradient cermet composites would be excellent candidates for cutting tool materials.     

Preparation and thermal decomposition of 5Mg(OH)2·MgSO4·2H2O nanowhiskers

Magnesium hydroxide sulfate hydrate (MHSH) nanowhiskers were prepared using magnesium chloride, ammonia and magnesium sulfate as raw materials by hydrothermal synthesis without any additional template. X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and thermal analysis (TG-DTA) were employed to characterize the composition and structural features of the MHSH nanowhiskers. It is shown that the thermal decomposition of nanowhiskers followed a three-step scheme. Based on DTA data, the reaction order, activation energy and pre-exponential factor for each step were calculated using a non-isothermal Kissinger method. It is also indicated from Satava method that the first step of the thermal decomposition of nanowhiskers is an A₂ nucleus formation and growth mechanism with integral form of G(a) = [−ln(1 − a)]^1/2. The second step is an A_u branching nuclei mechanism with integral form of G(a) = ln[a/(1 − a)], and the final step is a P_1/2 nucleation mechanism with integral form of G(a) = a^1/2.     

Effects of normal load on nanotribological properties of sputtered carbon nitride films

The nanotribological properties of amorphous carbon nitride (CN_x) films of ∼380 nm thickness were investigated, in the normal (contact) load range of 2–20 mN, using a Berkovich diamond indenter. The amorphous CN_x films tested in this work were grown on Si(100) substrates by reactive sputtering and energetic ion bombardment during deposition (IBD). The dependence of the friction behavior of the CN_x films on normal load (NL) was investigated in terms of nanomechanical properties, deformation mode and Atomic Force Microscopy (AFM) images of scratched surfaces, and the intensity of IBD. In films sputtered without IBD, the increase of the normal load caused the coefficient of friction to decrease initially to a minimum value and, subsequently, to increase to a maximum value, after which, it remained constant. The dominant friction mechanism in the low-load range was adhesion, while both adhesion and ploughing mechanisms contributed to the friction behavior in the intermediate and high-load ranges. Elastic and plastic deformation (PD) and delamination of the amorphous CN_x films occurred, depending on the normal-load ranges. On the other hand, films sputtered with high-energy IBD showed a load-dependent transition in both the scratch and the friction responses. Nanoscratching below 5 mN showed mainly elastic behavior of the film, while above 10 mN, a mixed elastic–plastic behavior was identified. Testing under a normal load of 20 mN resulted in local grooving at the film surface; however, in situ profiling of the scratch trace and AFM images showed no evidence of film failure. The increased load-carrying capacity, higher hardness and elastic response obtained with films grown with high-energy IBD, and the dominant friction mechanism at each load range illustrate the normal load dependence of the nanotribological properties of the sputtered CN_x films.