ENSURING TIGHTNESS IN PRESSURE COUPLING PARTS

Work objective is to solve the urgent problem of increasing the tightness and reliability of pressure couplings during their operation under dynamic loads. Theoretical and experimental studies assessing the impact on the tightness due to roughness nature of mating surfaces and three types of coatings: soft, double-layer and hard have been undertaken. The joints were tested under the influence of axial cyclic load and torque on a bench for accelerated testing. It is established that tightness of pressure couplings during operation under dynamic loads significantly depends on the parameters of microgeometry and physical and mechanical properties of the mating surface material that determine their actual contact area. Recommendations for preparing the surfaces of parts before pressure coupling assembling have been developed. It is proved that the use of regular microrelief and soft galvanic coatings of mating surfaces have a significant effect on the tightness of pressure couplings.

Test and Theory for a Refined Structural Model of a Hirth Coupling

Hirth coupling transmits high torques in the rotating assemblies of compressors and turbines. Their mating surface contacts cause local changes in lateral shaft stiffness. This is affected by the teeth geometry, contact surface area, coupling preload, and surface finish at the contact faces. Industry practice ignores localized lateral flexibility from the Hirth coupling, or is guided by limited experience-based rules of thumb. The authors provide a novel modeling approach utilizing 3D solid finite elements which accounts for contact deformations, intricate interface teeth geometries, stress concentration, and surface finish. This provides an increased accuracy localized stiffness model for the Hirth coupling, to improve rotordynamic response predictions. Free-free natural frequencies of a test rotor including a Hirth coupling are experimentally measured. The rotor is instrumented with strain gauges for preload force measurements, and the Hirth coupling contacting surface profiles are measured with a stylus type surface profiler. A GW contact model is obtained from the measured surface profiles. An iterative computation algorithm is utilized to calculate Hirth coupling contact stiffness and contact pressure at the complex-shaped contact surfaces. Predicted and measured natural frequencies are compared vs. preload.

Test and Theory for a Refined Structural Model of a Hirth Coupling

Hirth coupling transmits high torques in the rotating assemblies of compressors and turbines. Their mating surface contacts cause local changes in lateral shaft stiffness. This is affected by the teeth geometry, contact surface area, coupling preload, and surface finish at the contact faces. Industry practice ignores localized lateral flexibility from the Hirth coupling, or is guided by limited experience-based rules of thumb. The authors provide a novel modeling approach utilizing 3D solid finite elements which accounts for contact deformations, intricate interface teeth geometries, stress concentration, and surface finish. This provides an increased accuracy localized stiffness model for the Hirth coupling, to improve rotordynamic response predictions. Free-free natural frequencies of a test rotor including a Hirth coupling are experimentally measured. The rotor is instrumented with strain gauges for preload force measurements, and the Hirth coupling contacting surface profiles are measured with a stylus type surface profiler. A GW contact model is obtained from the measured surface profiles. An iterative computation algorithm is utilized to calculate Hirth coupling contact stiffness and contact pressure at the complex-shaped contact surfaces. Predicted and measured natural frequencies are compared vs. preload.

A High Precision Modeling Technology of Material Surface Microtopography and Its Influence on Interface Mechanical Properties

In order to accurately and effectively obtain the contact performance of the mating surface under the material surface topography characteristics, a numerical simulation method of rough surface based on the real topography characteristics and a multi-scale hierarchical algorithm of contact performance is studied in this paper. Firstly, the surface topography information of materials processed by different methods was obtained and characterized by a measuring equipment; Secondly, a non-Gaussian model considering kurtosis and skewness was established by Johnson transform based on Gaussian theory, and a rough surface digital simulation method based on real surface topography was formed; Thirdly, a multi-scale hierarchical algorithm is given to calculate the contact performance of different mating surfaces; Finally, taking the aeroengine rotor as the object, the non-Gaussian simulation method was used to simulate the mating surfaces with different topographies, and the multi-scale hierarchical algorithm was used to calculate the contact performance of different mating surfaces. Analysis results showed that the normal contact stiffness and elastic–plastic contact area between the mating surfaces of assembly 1 and assembly 2 are quite different, which further verifies the feasibility of the method. The contents of this paper allow to perform the fast and effective calculation of the mechanical properties of the mating surface, and provide a certain analysis basis for improving the surface microtopography characteristics of materials and the product performance.

Surface Modification Using Polydopamine-Coated Liquid Metal Nanocapsules for Improving Performance of Graphene Paper-Based Thermal Interface Materials

Given the thermal management problem aroused by increasing power densities of electronic components in the system, graphene-based papers have raised considerable interest for applications as thermal interface materials (TIMs) to solve interfacial heat transfer issues. Significant research efforts have focused on enhancing the through-plane thermal conductivity of graphene paper; however, for practical thermal management applications, reducing the thermal contact resistance between graphene paper and the mating surface is also a challenge to be addressed. Here, a strategy aimed at reducing the thermal contact resistance between graphene paper and the mating surface to realize enhanced heat dissipation was demonstrated. For this, graphene paper was decorated with polydopamine EGaIn nanocapsules using a facile dip-coating process. In practical TIM application, there was a decrease in the thermal contact resistance between the TIMs and mating surface after decoration (from 46 to 15 K mm2 W−1), which enabled the decorated paper to realize a 26% enhancement of cooling efficiency compared with the case without decoration. This demonstrated that this method is a promising route to enhance the heat dissipation capacity of graphene-based TIMs for practical electronic cooling applications.

Novel three-body nano-abrasive wear mechanism

Current three-body abrasive wear theories are based on a macroscale abrasive indentation process, and these theories claim that material wear cannot be achieved without damaging the hard mating surface. In this study, the process of three-body nano-abrasive wear of a system including a single crystalline silicon substrate, an amorphous silica cluster, and a polyurethane pad, based on a chemical mechanical polishing (CMP) process, is investigated via molecular dynamics simulations. The cluster slid in a suspended state in smooth regions and underwent rolling impact in the asperity regions of the silicon surface, realizing non-damaging monoatomic material removal. This proves that indentation-plowing is not necessary when performing CMP material removal. Therefore, a non-indentation rolling-sliding adhesion theory for three-body nano-abrasive wear between ultrasoft/hard mating surfaces is proposed. This wear theory not only unifies current mainstream CMP material removal theories, but also clarifies that monoatomic material wear without damage can be realized when the indentation depth is less than zero, thereby perfecting the relationship between material wear and surface damage. These results provide new understanding regarding the CMP microscopic material removal mechanism as well as new research avenues for three-body abrasive wear theory at the monoatomic scale.

Microstructure and Wear Behavior of Plasma-Sprayed TiO2–SiAlON Ceramic Coating

In this study, atmospheric plasma spray was employed to deposit TiO2–SiAlON ceramic coating on 316 stainless steel. The phases and microstructure of the ceramic coating were investigated. Additionally, comparative studies on the tribological performances of the substrate and the ceramic coating, under both dry and starved lubrication conditions, were carried out. The SiAlON phase was preserved, while partial TiO2 anatase was transformed to rutile phase. The wear rate of the coating was roughly 1/3 of that of the substrate under both conditions. The wear mechanisms of the ceramic coating were surface fracture and abrasive wear in both cases, and the coating under starved lubrication underwent less abrasion. The pores in the coating served as micro-reservoirs, forming an oil layer on the mating surface, and improving tribological properties during sliding.

Optimization of Selective Assembly for Shafts and Holes Based on Relative Entropy and Dynamic Programming

Selective assembly is the method of obtaining high precision assemblies from relatively low precision components. For precision instruments, the geometric error on mating surface is an important factor affecting assembly accuracy. Different from the traditional selective assembly method, this paper proposes an optimization method of selective assembly for shafts and holes based on relative entropy and dynamic programming. In this method, relative entropy is applied to evaluate the clearance uniformity between shafts and holes, and dynamic programming is used to optimize selective assembly of batches of shafts and holes. In this paper, the case studied has 8 shafts and 20 holes, which need to be assembled into 8 products. The results show that optimal combinations are selected, which provide new insights into selective assembly optimization and lay the foundation for selective assembly of multi-batch precision parts.

Physico-mechanical, thermal, and tribological studies of polytetrafluoroethylene-filled polyoxymethylene/silicone composites

Polyoxymethylene (POM)-based composites with polytetrafluoroethylene (PTFE) filler and silicone gum have been prepared by melt extrusion to enhance the wear resistance and friction lubrication of POM without compromising the other desired properties such as modulus, toughness/impact strength, notch insensitivity, and thermal stability. The compounded material was injection molded to prepare test specimens, and their physico-mechanical properties were evaluated. In addition, thermal and tribological characteristics of the composites were also studied. The addition of silicone into POM/PTFE composites could enhance the formation of stable transfer film on the mating surface during sliding contact, thus improving the friction and wear performance, as silicone forms synergistic mixture with PTFE. It was found that the tensile, flexural, and notched impact strength remained almost constant for all the formulations. The use of PTFE improved the unnotched impact strength (from 35.5 to 42.9 kJ m−2). The toughening effect can be attributed to the dissipation of impact energy through soft PTFE and ductile silicone phase. Differential scanning calorimeter results revealed that there are no negative effects on POM crystallinity due to the presence of PTFE and silicone. The wear behavior of composites has been investigated under dry sliding conditions at different normal loads and sliding velocities at room temperature. The POM/PTFE/silicone (90/8/2 wt/wt%) formulation exhibits better wear-resistant behavior in the present study.