Good Wear Protection Research Articles

Generation of protective ceramic coatings on magnesium by laser treatment of Al2O3/ZrO2 filled organosilazane

Protective ceramic coatings are frequently the most common solutions for problems like corrosion and wear. Polymer derived ceramics technology is suitable for the preparation of ceramic coatings by pyrolysis in a furnace but requires high temperatures for the ceramization. To overcome this restriction a Nd:YVO4 laser (λ = 1064 nm) as energy source was used to process a novel ceramic coating system composed of an organosilazane with zirconia and alumina particles specially to protect magnesium. The resulting dense ceramic coating with a thickness up to 20 µm exhibits very good adhesion (25.9 ± 2.7 MPa) to the magnesium substrate, exceptionally high hardness (19.79 ± 1.90 GPa), an outstanding thermal shock stability but also a low coefficient of friction (0.21 ± 0.02) and a very good wear protection towards a SiC counter body. Furthermore, the coating protects the magnesium substrate effectively against corrosion (5 wt% aqueous NaCl solution).

The Oxide Layer of 10Mn5 Medium Manganese Steel for Wear Protection in High-Temperature Friction during Hot Stamping

A custom-designed high-temperature sliding-on-sheet-strip (SOSS) tribo-tester was used to simulate the high-temperature friction process of 10Mn5 medium manganese steel bare plate under actual hot stamping conditions. To reveal its high-temperature friction mechanism in the hot forming process, the high-temperature friction behavior of 10Mn5 steel and 22MnB5 steel was compared. The scanning electron microscope (SEM), energy spectrum analyzer (EDS) and X-ray diffractometer (XRD) were used to investigate the structure of the oxide layer, composition of physical phase, wear surface morphology and elemental composition. The results show that the average coefficient of friction of 10Mn5 steel is 12.7% lower than that of 22MnB5 steel. The cross-section of both steel consists of an oxide layer, an alloying element-rich layer and the matrix. The oxide layer of 10Mn5 steel is mainly composed of Fe3O4, approximately 63.7%, while that of 22MnB5 is mainly composed of Fe2O3, approximately 66.9%. The complete and less flaking scale of 10Mn5 steel provides good wear protection, and the mechanism is abrasive with slight adhesive wear. Meanwhile, oxide particles and fragments are embedded in the 22MnB5 surface thus increasing the wear, and the mechanism evolves into severe abrasive and adhesive wear. The difference in the mechanism between the two steels is mainly caused by different austenitizing temperatures, which for 10Mn5 is lower than 22MnB5, about 100 °C. This makes the thermal stress of 10Mn5 from the temperature difference between the furnace and the environment not enough to break the scale and decrease abrasion.

Alkylated Polyphenyl Ethers as High-Performance Synthetic Lubricants

Lubricants exhibiting both thermal and chemical stability that consequently generate less hydrogen during friction are required to avoid the hydrogen embrittlement of moving mechanical components. The present work studied the effects of the length and number of alkyl chains on the tribological properties of polyphenyl ethers (PPEs), which feature good thermal and radiation resistance. PPEs were found to have much lower friction coefficients compared with a poly-alpha-olefin and alkyldiphenyl ether, and the effect of the running-in process on friction appeared to be negligible. The formation of polymers on the friction track evidently decreased the friction coefficients of the lubricants and the wear rates were almost zero for all the PPEs, indicating excellent anti-wear properties. Analyses with a quadrupole mass spectrometer connected to a friction tester under vacuum indicated negligible hydrogen generation from 4P2E, which had no alkyl chains, after the running-in. R1-4P2E, having a C16H33 chain, showed hydrogen desorption similar to that of the alkyldiphenyl ether, which had a C18H37 alkyl chain. R2-4P2E, with two C16H33 chains, produced significant hydrogen, but with a long induction period; thus, it provided good wear protection. Although alkyl chains increased the risk of hydrogen generation, PPEs with such chains may have applications as standard lubricants.

Corrosion Properties of Ni-P-B Dispersion Coating for Industrial Knives and Blades

Industrial blades, particularly the one used in food processing, requires not only a high hardness and a good wear protection but also an adequate corrosion protection. Currently, there is a wide variety of materials being used for this purpose that satisfy the above requirements. However, they come at a cost. Typically, steel with a high number of alloyed elements is chosen because the alloyed elements improved the steel quality. For example, incorporation of nickel, chromium, phosphorus and molybdenum in the steel improves amongst others the corrosion protection. However, the thermal processing of such steel, that is quenching, repeated tempering and followed by nitriding or boriding, is lengthy and complicated which drive the cost to produce knives using such steel up.Besides steel cutting tools, there is also cutting tools made from ceramic and carbide. Compared to steel knives, carbide knives have a longer lifespan and provide a higher temperature tolerance, which results in the application of the carbide cutting tools at higher speed and for longer periods without experiencing tool failure. Admittedly, due to carbides high hardness and thus the increase in difficulty to machine carbide, the production cost of carbide cutting tools is much higher compared to their steel counterparts. For ceramics, while the material is corrosion free and can maintain hardness and wear properties at a very high temperature, it is also more brittle than steel and carbide cutting tools. This results in premature chipping of the cutting edge and a shorter lifespan of the blade.To further reducing the production cost of industrial knives, the alternative of using a low alloyed steel which is coated with a hard coating and adjacently thermally treated for 1 h is investigated. This method is chosen because the tempering process of the low alloyed steel is not as lengthy as for high alloyed steel. The homogenously incorporated boron particles in the nickel phosphorus coating reduce the subsequent thermal treatment duration due to a shorter diffusion path compared to the conventional boriding. Previous study by the author [1] shows this approach to produce a high hardness at approx. 900 HV. Electroless nickel phosphorus coating is applied due to its good anti-corrosive properties. The presence of boron as particles in the coating or as boride after thermal treatment could change the corrosion behaviour of the coating. To date, no studies have yet been done to determine its corrosion properties and benchmarking this Ni-P-B dispersion coating with the other coating systems.In this study, the corrosion behaviour of the dispersion layer as coated and after thermal treatments at different temperatures is investigated. The corrosion resistance in NaCl 3.5 % is characterized though potentiodynamic polarisation and electrochemical impedance spectrometry. The results will be evaluated and compared to the other established coating systems.[1] The previous study will be presented at 241st ECS Meeting in Vancouver, Canada and has not yet been published at the time this abstract is submitted for the 242nd ECS Meeting in Atlanta.

Thermally sprayed coatings for protection of integrated sensor systems on tribological loaded surfaces

Piezo-element integration in WC-NiCr and Cr3C2-NiCr thermally sprayed coatings resulted in electric voltage curves, which can provide information about the wear and friction condition in the tribological contact zone. The coatings were produced by high-velocity oxy fuel (HVOF) spraying with a thickness of 350 μm. The thermal sprayed coatings provided good wear protection of the embedded piezo-elements. The electrical energy outcome was slightly higher for the more wear resistant WC-NiCr coatings under dry sliding with a WC-12Co ball. The Cr3C2-NiCr coatings are known to give a better wear resistance at higher temperatures, but exhibit an inferior behaviour in comparison to WC-NiCr coatings at room temperature. This tendency was observed and is very well mirrored by the shape of electric voltage and the electric signal output at room temperature. Piezo-element integration was carried out after HVOF spraying embedding the 200 μm thick piezo-elements into cross-sectional slit which was produced by wire erosion.

Influence of temperature on wear performance of greases in rolling bearings

Purpose Rolling bearing operation under mixed and boundary lubrication conditions may lead to heavy adhesive or abrasive wear, which may lead to wear-induced rolling bearing failure. The purpose of this paper is to investigate the wear protection capabilities of different grease compositions at varying temperatures. It is considered that the temperature influences the lubrication conditions, the behaviour of grease components, namely, bleed oil and thickener, as well as the tribofilm formation due to tribo-chemical interactions between additives and surfaces. Design/methodology/approach In this study, four different greases were produced on the basis of a mineral base oil by varying the thickener and the addition of ZDDP. Various grease-lubricated rolling bearing experiments were conducted in a wide temperature range from 0°C to 120°C. Subsequently, the wear pattern, tribofilm formation and grease structures were analysed. Thereby, the influence of the different grease thickeners and the performance of ZDDP as a common antiwear and extreme pressure additive was evaluated. Findings The results show a strong temperature-dependency and allow a classification of temperature ranges concerning wear protection. At low temperatures, all greases provide a very good wear protection without the evidence of additive-based tribofilm formation. In the experiments at elevated temperatures, ZDDP tribofilms were formed. The formation depends on the thickener type: in comparison to lithium thickener, polyurea thickener favours more protective tribofilms at the same temperature. The experimental results show that medium temperatures in the range of 40°C–60°C are critical concerning wear due to the insufficient tribolayer formation and limited load carrying capacity of the grease. Originality/value Temperature is a key operating parameter for grease lubrication in roller bearings. The experimental work enables consideration of different impact pathways of temperature by combining roller bearing tests and microanalysis.

Synthesis and Characterization of Trimethylolpropane Based Esters as Green Biolubricant Basestock

Bio-based lubricant is crucial to be developed considering the toxicity risk, climate change, energy security, and green-environmental approach. In this work, trioleate trimethylolpropane (TOTMP) (1) was synthesized through the esterification between trimethylolpropane (TMP) and oleic acid (OA) using a sulfuric acid catalyst. TOTMP was chemically modified to produce hyperbranched nonaoleate trimethylolpropane (NOTMP) (4). TOTMP was first epoxidized to epoxidized trioleate trimethylolpropane (ETOTMP) (2) followed by ring-opening reaction to produce trihydroxyhexaoleate trimethylolpropane (THHOTMP) (3). Lastly, THHOTMP was further esterified with OA to produce hyperbranched biolubricant NOTMP. Chemical structure confirmation of each product was performed by using Fourier transformation infrared (FTIR), proton, and carbon nuclear magnetic resonance (1H and 13C NMR). The physicochemical analysis showed that the oxidative stability of NOTMP was at 172°C, pour point at -34°C, flash point at 320°C, and viscosity index at 237. The tribology analysis showed good wear protection with a minimal decrease in the friction coefficient (µ). Furthermore, rheology analysis showed that all synthesized compounds had been classified as Newtonian fluids. Therefore all the branched esters' bio lubricant products were successfully produced with improved lubricity properties, which are plausible for bio lubricants for both tropical and temperate countries.

Graphite-based solid lubricant for high-temperature lubrication

High-temperature solid lubricants play a significant role in the hot metal forming process. However, preparing high-temperature solid lubricant is formidably challenging due to the stern working conditions. Here we successfully develop a new type of eco-friendly high-temperature graphite-based solid lubricant by using amorphous silica dioxide, aluminum dihydrogen phosphate, and solid lubricant graphite. The solid lubricating coating exhibits excellent tribological properties with a very low friction coefficient and good wear protection for workpiece at high temperature under the air atmosphere. An array of analytical techniques reveals the existence of solid lubricant graphite in the lubricating coating after the high-temperature friction test. A synergistic effect between the protective surface film and the solid lubricant graphite is proposed to account for such superior lubricating performance. This work highlights the synergistic effect between the protection layer and the lubricant graphite and further provides the insight in designing the high-temperature solid lubricant.

Wear and Corrosion Resistance of Fe-Based Coatings Reinforced by TiC Particles for Application in Hydraulic Systems

Thermally sprayed Fe-based coatings reinforced by TiC particles are a cost-effective alternative to carbide coatings such as WC/CoCr, Cr3C2/NiCr, and hard chrome coatings. They feature a good wear resistance and—with sufficient amount of alloying elements like Cr and Ni—also a high corrosion resistance. In hydraulic systems, the piston is usually coated with hard chrome coatings for protection against corrosion and wear. New water-based hydraulic fluids require an adaption of the coating system. In order to investigate the wear and corrosion resistance of Fe/TiC, a novel powder consisting of a FeCr27Ni18Mo3 matrix and 34 wt.% TiC was applied by HVOF and compared to reference samples made of WC/CoCr (HVAF) and hard chrome. Besides an in-depth coating characterization (metallographic analyses, electron microprobe analyzer–EMPA), wear resistance was tested under reverse sliding in a water-based hydraulic fluid. The novel Fe/TiC coatings showed good wear protection properties, which are comparable to conventional coatings like WC/CoCr (HVAF) and electroplated hard chrome coatings. Corrosion resistance was determined by polarization in application-oriented electrolytes (hydraulic fluid at 60 °C, artificial sea water at RT). The corrosion resistance of the investigated iron-based coatings at 60 °C was superior to the references coatings for both hydraulic fluids. Selected coatings were tested in an application-oriented hydraulic test bench with HFC hydraulic fluid (water polymer solutions) showing comparably good wear and corrosion resistance as the hard chrome-coated reference.

Tribological properties of Mg/Al–CO3 layered double hydroxide as additive in base oil

The tribological performance of Mg/Al–CO3–layered double hydroxide (LDHs) nanoparticles in base oil was studied as a sole additive and in combination with zinc dialkyldithiophosphate (ZDDP) at 100°C. Results show that LDHs could improve antifriction and antiwear properties of base oil. The blend with LDHs alone shows better antifriction properties than that of the mixture of LDHs and ZDDP, but the addition of ZDDP helps to stabilise the friction coefficient. Surface analyses have been performed to study the morphology, nanoparticle distribution and chemical species in the tribofilm. The results show that tribofilms contain Mg, Al, C, O, Zn, P and S, and the structure of Mg/Al–CO3–LDHs changes under shearing stress and this process can help reduce friction coefficient. Thus, LDHs could be chemically incorporated into tribofilm to reduce the polyphosphate chain length originated from ZDDP decomposition resulting in a medium chain polyphosphate, which provides very good wear protection.

Effects of molecular weight of grafted hyaluronic acid on wear initiation

Hyaluronic acid (HA) of different molecular weights (Mw) was grafted onto mica surfaces to study the effects of Mw on the conformation and wear protection properties of a grafted HA (gHA) layer in lubricin (LUB) and bovine synovial fluid (BSF) using a surface forces apparatus. The Mw of gHA had significant effects on the wear pressure (Pw), at which point the wear initiates. Increasing the gHA Mw from 51 to 2590kDa increased Pw from 4 to 8MPa in LUB and from 15 to 31MPa in BSF. The 2590kDa gHA in BSF had the best wear protection (Pw∼31MPa), even though it exhibited the highest friction coefficient (μ∼0.35), indicating that a low μ does not necessarily result in good wear protection, as is often assumed. The normal force profile indicated that BSF confines the gHA structure, making it polymer brush-like, commonly considered as an excellent structure for boundary lubrication.

Cracking Resistance and Impact Wear of Thin and Thick Hard Coatings Under Cyclic Loading

Abstract Among wear resistant coatings and deposition techniques physical vapour deposition of hard thin coatings and high-velocity oxy-fuel spraying of thick WC-Co-based powder coating have provided good wear protection against various types of wear including impact wear. Three different substrate and coating systems—hard metal-coating, cold work tool steel-coating and nitrided steel-coating—were studied. Mono- and multilayer coatings as well as duplex coatings were selected for testing. For characterization of coating systems nanohardness and microhardness as well as Rockwell adhesion test and single point indentation methods were used. Wear resistance of coatings was determined considering impact wear. The best adhesion determined by indentation was demonstrated by monolayer coating (TiN) on a hard metal substrate; the same coating system has a higher impact wear resistance. The coatings with multilayer structure (TiAlN) demonstrate better cracking resistance. Resistance to cracking as well as to impact wear depends on coating E/H ratio (at lower loads) as well as on substrate material E/H ratio (at higher loads). For characterization of resistance to plastic deformation for substrate Hs3Es2 ratio was used. The obtained data enable a comparison of cracking and impact wear resistance of a coated system and selection of an optimal coating for metal forming tools applications.

The Boundary Lubrication of Chemically Grafted and Cross-Linked Hyaluronic Acid in Phosphate Buffered Saline and Lipid Solutions Measured by the Surface Forces Apparatus

High molecular weight hyaluronic acid (HA) is present in articular joints and synovial fluid at high concentrations; yet despite numerous studies, the role of HA in joint lubrication is still not clear. Free HA in solution does not appear to be a good lubricant, being negatively charged and therefore repelled from most biological, including cartilage, surfaces. Recent enzymatic experiments suggested that mechanically or physically (rather than chemically) trapped HA could function as an "adaptive" or "emergency" boundary lubricant to eliminate wear damage in shearing cartilage surfaces. In this work, HA was chemically grafted to a layer of self-assembled amino-propyl-triethoxy-silane (APTES) on mica and then cross-linked. The boundary lubrication behavior of APTES and of chemically grafted and cross-linked HA in both electrolyte and lipid 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) solutions was tested with a surface forces apparatus (SFA). Despite the high coefficient of friction (COF) of μ ≈ 0.50, the chemically grafted HA gel significantly improved the lubrication behavior of HA, particularly the wear resistance, in comparison to free HA. Adding more DOPC lipid to the solution did not improve the lubrication of the chemically grafted and cross-linked HA layer. Damage of the underlying mica surface became visible at higher loads (pressure >2 MPa) after prolonged sliding times. It has generally been assumed that damage caused by or during sliding, also known as "abrasive friction", which is the main biomedical/clinical/morphological manifestation of arthritis, is due to a high friction force and, therefore, a large COF, and that to prevent surface damage or wear (abrasion) one should therefore aim to reduce the COF, which has been the traditional focus of basic research in biolubrication, particularly in cartilage and joint lubrication. Here we combine our results with previous ones on grafted and cross-linked HA on lipid bilayers, and lubricin-mediated lubrication, and conclude that for cartilage surfaces, a high COF can be associated with good wear protection, while a low COF can have poor wear resistance. Both of these properties depend on how the lubricating molecules are attached to and organized at the surfaces, as well as the structure and mechanical, viscoelastic, elastic, and physical properties of the surfaces, but the two phenomena are not directly or simply related. We also conclude that to provide both the low COF and good wear protection of joints under physiological conditions, some or all of the four major components of joints-HA, lubricin, lipids, and the cartilage fibrils-must act synergistically in ways (physisorbed, chemisorbed, grafted and/or cross-linked) that are still to be determined.

Effect of the Substrate Geometry on Plasma Synthesis of DLC Coatings

AbstractDiamond like carbon (DLC) coatings provide very good wear protection in application with machine elements like bearings, gears, shafts and chains. Ensuring the coating characteristics and reproducibility of DLC coatings on different geometries are major concerns in industrial application scale. Due to the change in the plasma intensity, the same coating process can result in different mechanical properties on different geometrical substrate shapes. In order to determine the influence of substrate geometry on the coating characteristics the plasma parameters, such as electron temperature and electron density should be determined. This study presents the effect of the substrate geometry on the plasma synthesis of hydrogen free (a‐C) and hydrogen containing (a‐C:H) DLC coatings deposited by magnetron sputter ion plating (MSIP) technology in an industrial coating facility. The electron temperature, plasma potential and electron density are determined by a Langmuir probe at two different substrate geometries, such that one of them has a planar and the other one has an angular surface, which corresponds to the geometries of a bearing and a gear, respectively. The adhesion and the mechanical characteristics, i.e. universal hardness and Young's modulus, of the DLC coatings are determined by scratch test and nanoindentation, respectively. The dependency of the mechanical characteristics on the plasma parameters for different geometrical surfaces is examined and the results are compared. The results show that depending on the plasma gas, either pure argon or mixture of argon and acetylene, the substrate geometry has a significant effect on mechanical properties and adhesion.

Adhesion and Stable Low Friction Provided by a Subnanometer-Thick Monolayer of a Natural Polysaccharide

Using a surface forces apparatus, we have investigated the adhesive and lubrication forces of mica surfaces separated by a molecularly thin, subnanometer film of a high-molecular-weight (2.3 MDa) anionic polysaccharide from the algae Porphyridium sp. adsorbed from aqueous solution. The adhesion and friction forces of the confined biopolymer were monitored as a function of time, shearing distance, and driving velocity under a large range of compressive loads (pressures). Although the thickness of the dilute polysaccharide was <1 nm, the friction was low (coefficient of friction = 0.015), and no wear was ever observed even at a pressure of 110 atm over 3 decades of velocity, so long as the shearing distances were less than twice the contact diameter. Atomic force microscopy in solution shows that the biopolymer is able to adsorb to the mica surface but remains mobile and easily dragged upon shearing. The adhesion (adsorption) of this polysaccharide even to negatively charged surfaces, its stable low friction, its robustness (high-load carrying capacity and good wear protection), and the weak (logarithmic) dependence of the friction force on the sliding velocity make this class of polyelectrolytes excellent candidates for use in water-based lubricant fluids and as potential additives to synovial fluid in joints and other biolubricating fluids. The physical reasons for the remarkable tribological properties of the ultrathin polysaccharide monolayer are discussed and appear to be quite different from those of other polyelectrolytes and proteins that act as thick "polymer brush" layers.

Tool wear mechanisms in intermittent cutting of metal matrix composites

Cutting of fibre and particle reinforced light alloys is known to cause extensive wear of cutting tool edges. With the objective to identify the mechanisms responsible for this, single edge cutting experiments on 20% alumina fibre reinforced SS 4103 and 30% SiC particle reinforced SS 4244 were performed. A modified Sharpy impact strength tester (the Uppsala Pendulum) was used to conduct the experiments. This set-up provides an excellent quick-stop facility which was used to freeze the cutting action. The tools were used for one single, quick-stopped, cut only. Then both tool and work material were studied in the scanning electron microscope. As expected, the hard reinforcing phases in both composites were found to cause severe abrasive wear on the tool. Contrary to the case when cutting ordinary, unreinforced metals and alloys, distinctive traces of single abrasive tool wear events were found on the cut surfaces of the composites. However, the whiskers reinforced alloy caused considerably less wear on the rake face than on the flank face of the tool. This was explained by the whiskers fragmenting when passing the primary shear zone. Thus, only smaller whisker fragments, less firmly situated in the matrix, reached the rake face. An experiment with diamond coated cemented carbide tools was also performed. The coatings showed no signs of abrasive wear but failed by spalling off the rake face at the moment of quick-stop. It was concluded that diamond coatings offers very good wear protection, provided that the adhesion between the tool and coating is strong enough.

Effects of Synthetic Industrial Fluids on Ball Bearing Performance

A series of laboratory tests was conducted using rolling contact bearings to compare the lubricating properties of a specially formulated synthetic lubricant based on diester to those of a specific mineral oil currently in service in petrochemical plant equipment. Two synthetic lubricants and two mineral oils of varying viscosities were experimentally compared. The test results indicated that the synthetic lubricant, having a viscosity of 32 m2 / s (32 cSt) at a temperature of 313K, offered longterm contact surface protection equivalent to that of the base line mineral oil which has a viscosity of 68 m2 / s, without reducing bearing service life below the theoretically predicted levels. The same good wear protection could not be achieved with a reduced viscosity mineral oil. The use of the lower viscosity synthetic lubricating fluid could provide projected energy savings of $75,000 per year when elements of a total petrochemical complex are considered.

Auger electron spectroscopy study on reaction layers formed under Reichert wear test conditions in the presence of extreme pressure additives

Auger electron spectroscopy depth profile analysis was used to study the reaction layers formed under Reichert wear test conditions in the presence of four different extreme pressure additives and hence to try to determine the wear prevention mechanism. Wear protection on Reichert wear test specimens was found to correlate with the formation of a thin, sulphur-rich reaction layer in the worn surface. Under FZG gear test conditions, phosphorus in the reaction layer appeared to be necessary for good wear protection

Reactively sputtered TiN coatings for tribological applications

The results of tribological studies carried out on TiN sputter-coated ferrous and non-ferrous materials are presented. A disc magnetron system was used to sputter TiN reactively onto samples suitable for standard friction and wear tests. The tests carried out show that thin films of reactively sputtered TiN, typically 5 microm thick, provide good wear protection for ferrous and non-ferrous surfaces subjected to sliding and rolling contacts. Cutting tests with coated cermet tool inserts show that reactive sputtering is also useful for improving the performance of cutting tool inserts. The results obtained suggest that magnetron reactive sputtering can be useful to enhance the wear resistance of finished precision components.

The design of the Fort St. Vrain steam generators

The steam generators for the Fort St. Vrain nuclear power plant are the first application in the United States of once-through boiler design coupled with a high temperature gas-cooled reactor. They contain many design features which are unique for this type of component. Since they are an integral part of the primary system and completely enclosed by the prestressed concrete reactor vessel, they must be removable as well as fit the space available for penetrations. These requirements made the once-through boiler principle a logical choice. Multi-start helically wound tubes supported by perforated plates in a star-shape arrangement resulted in an extremely compact design. The helium inlet temperature of 1427°F and steam temperatures of 1005°F main and 1001°F reheat required unique solutions in terms of flexibility and cooling of support systems and selection of insulation materials and design. Operation in a helium atmosphere without a protective oxide layer called for materials with good wear protection characteristics where parts may experience relative motion. Stabilizing orifices, externally adjustable at the steam generator inlet, plus essentially equal tube lengths for each of the many parallel circuits are utilized to balance circuit performance. To minimize gas bypass flows, special gas seals are provided around individual tube bundles. Field erection time was minimized by developing an upper and a lower module assembly and joining them after erection in the reactor vessel.