Pack Cementation Research Articles

Effect of the Activator Type on the Microstructure and Properties of the Vanadizing Layer on the Surface of GCr15 Steel

This research aims to replace the activator NH4Cl in a vanadizing agent and reduce air pollution and harm to the human body. This study adopts powder pack cementation to prepare the vanadizing layer on the surface of GCr15 steel, focusing on the influence of the activator types (NH4Cl, CuCl, CuCl+Cr powder, and CuCl+Ni powder) on the structure and properties of the vanadizing layer on the surface of GCr15 steel. The results show that the vanadizing layer is prepared on the surface of GCr15 steel with different activators and that they are closely bonded to the matrix. The main phase composition of the vanadizing layer is VCx, and it has a preferred orientation on the (111) crystal surface. When the type of activator is NH4Cl, the prepared vanadizing layer has a lower friction factor. When the activator type is CuCl+Cr powder, the prepared vanadizing layer has less inclusions, the thickness of the vanadizing layer is the highest at 10.87 μm, and the hardness is the highest at 2331.7HV. Considering the thickness of the layer, the hardness, and the wear resistance, the best performance of the vanadizing layer was obtained when the activators were CuCl+Cr powder.

Ablation performance of Ta0.8Hf0.2C-SiC coating fabricated via pack cementation for carbon/carbon composites

Ta0.8Hf0.2C solid solution combines the excellent oxidation and ablation resistance of TaC and HfC, making it ideal for ultra-high temperature applications. Herein, a Ta0.8Hf0.2C-SiC coating was fabricated on carbon/carbon (C/C) composites via pack cementation, with the volume fraction of Ta0.8Hf0.2C being only 0.57%. The coating provided over 350seconds of thermal protection at 2.4MW/m2 (2150 ℃) and 170seconds at 6.2MW/m2 (2400 ℃) for C/C composites, demonstrating exceptional ablation resistance with low addition of Ta0.8Hf0.2C. The mass and linear ablation rates were only 0.5110mg/s and 0.0277 μm/s at 2150 ℃, and 0.3947mg/s and 0.6229 μm/s at 2400 ℃, respectively. The solid solution structure of Ta0.8Hf0.2C ensures uniform atomic-scale distribution of Ta and Hf, which facilitaes the in-situ pinning of liquid-phase Ta2O5 by HfO2/Hf6Ta2O17 particles.

Study on the Oxidation Behavior of TiB2-CeO2-Modified (Nb,Mo,Cr,W)Si2 Coating on the Surface of Niobium Alloy.

A novel TiB2-CeO2-modified (Nb,Mo,Cr,W)Si2 coating was prepared on a Nb-5W-2Mo-1Zr alloy substrate using two-step slurry sintering and halide-activated pack cementation to address the limitations of a single NbSi2 coating in meeting the service requirements of niobium alloys at elevated temperatures. At 1700 °C, the static oxidation life of the coating exceeded 20 h, thus indicating excellent high-temperature oxidation resistance. This was due to the formation of a TiO2-SiO2-Cr2O3 composite oxide film on the coating surface, which, due to low oxygen permeability, effectively prevented the inward infiltration of oxygen. Additionally, the dense structure of the composite coating further enhanced this protective effect. The composite coating was able to withstand over 1600 thermal shock cycles from room temperature to 1700 °C, and its excellent thermal shock performance could be attributed to the formation of MoSi2, CrSi2, and WSi2 from elements such as Mo, Cr, and W, which were added during modification. In addition to adjusting the difference in thermal expansion coefficients between the layers of composite coatings to reduce the thermal stress generated by thermal shock cycles, the formation of silicide compounds also improved the overall fracture toughness of the coating and thereby improved its thermal shock resistance.

Three-point bending damage detection of SiC coated C/C composites based on acoustic emission

In this study, the three-point bending process of needled C/C composites, pack cementation SiC coated-C/C composites (PC-SiC-C/C) and chemical vapor deposition SiC coated-C/C composites (CVD-SiC-C/C) were monitored by the acoustic emission (AE) technology. To study the bending damage evolution process of three kinds of composite materials, the K-means algorithm was used to analyze the AE characteristic parameters. The damage type was analyzed by combining the electron microscope image of the sample fracture. The findings revealed that C/C composites and CVD-SiC-C/C composites contain three distinct types of damage: matrix cracking, carbon fiber-pyrolytic carbon interface debonding, and fiber fracture under three-point bending loading. PC-SiC-C/C composites has two additional damage: coating cracking and pyrolytic carbon-SiC interface debonding. The various damages could be identified by different AE signals: matrix cracking (175.781–421.875 KHz), carbon fiber-pyrolytic carbon interface debonding (70.312–164.062 KHz), fiber fracture (2.929–46.875 KHz), coating cracking (375–585.937 KHz), pyrolytic carbon-SiC interface debonding (169.921–257.812 KHz). The cumulative AE energy shows that the order of damage types of the three composites during the three-point bending loading is different.

Improvement of fatigue life by aluminizing of additive manufactured Fe- and Ni-base alloy

Laser powder bed fusion (PBF-LB/M) is used in various industries to manufacture complex parts with high precision. High surface roughness and porosity have a negative impact on fatigue resistance. This work presents aluminum pack cementation as a method to reduce surface roughness and improve fatigue resistance of AM parts. An Fe-base alloy (Alloy 800H) and a Ni-base alloy (Alloy 699XA) are selected. Some of the specimens made from PBF-LB/M were treated by pack cementation with aluminum, while others were subjected to the vibratory finishing process. The surface roughness, microhardness distribution and the microstructure of the interface zone were measured and analyzed for the as-built, vibratory finished and aluminized specimens. Rotating bending fatigue test was performed at room temperature. Conventionally fabricated specimens of both investigated alloys were tested under the same conditions to evaluate the effects of pack cementation. The aluminized samples (PBF-LB/M) show a significant reduction in surface roughness with a decrease of 45 % for Alloy 800H and 65 % for Alloy 699XA. This leads to an improvement in fatigue life for both Alloy 800H and Alloy 699XA. In contrast, the conventionally fabricated specimens exhibited increased surface roughness after pack cementation compared to their initial condition and showed a significant reduction in fatigue resistance.

Aluminide Coatings by Means of Slurry Application: A Low Cost, Versatile and Simple Technology

The present study focused on demonstrating the versatility of the slurry deposition technique to produce aluminide coatings to protect components from high-temperature corrosion in a broad temperature range, from 400 to 1400 °C. This is a simpler and low-cost coating technology used as an alternative to CVD and pack cementation, which also allows the coating of complex geometries and offers improved and simple repairability for a lot of industrial applications, along with avoiding the use of non-hazardous components. Slurry aluminide coatings from a proprietary water-based-Cr6+ free slurry were produced onto four different substrates: A516 carbon steel, 310H AC austenitic steel, Ti6246 Ti-based alloy and TZM, a Mo-based alloy. The resulting coatings were thoroughly characterised by FESEM and XRD, mainly so that the identification of microstructures and appropriate phases was reported for each coating. The importance of surface preparation and heat treatment as key parameters for the coating final microstructures was also evidenced, and how those parameters can be optimised to obtain stable intermetallic phases rich in Al to sustain the formation of a protective Al2O3 oxide scale. These coating systems have applications in diverse industrial environments in which high-temperature corrosion limits the lifetime of the components.

Ablation-resistant yttrium-modified high-entropy refractory metal silicide (NbMoTaW)Si2 coating for oxidizing environments up to 2100 °C

Refractory high-entropy alloys (RHEAs) are pivotal in ultra-high temperature applications, such as rocket nozzles, aerospace engines, and leading edges of hypersonic vehicles due to their exceptional mechanical ability to withstand severe thermal environments (in excess of 2000 °C). However, the selection of materials that satisfy the stringent criteria required for effective ablation resistance remains notably restricted. Here, a novel yttrium-modified high-entropy refractory metal silicide (Y-HERMS) coated on a refractory high-entropy NbMoTaW alloy is developed via pack cementation process. The developed Y-HERMS coating with sluggish diffusion effect demonstrates extraordinary ablation resistance, maintaining near-zero damage at sustained temperatures up to 2100 °C for a duration of 180 s, surpassing state-of-the-art high-performance silicide coatings. Such exceptional ultra-high ablation performance is primarily ascribed to the in-situ development of a high viscosity Si-Y-O oxide layer with increased thermal stability and the presence of high-melting Y(Nb0.5Ta0.5)O4 oxides as skeleton structure. Theoretical results elucidate that the Y-HERMS promotes the formation of SiO2, which impedes the diffusion of O into metal silicide layer, synergistically contributing to the superior ablation resistance. These findings highlight the potential of utilizing high-entropy materials with excellent ablation resistance in extreme thermal environments.

Protective silicide coating fabricated on ductile VNbTa refractory complex concentrated alloy: Microstructure, high-temperature oxidation and wear behaviors

VNbTa refractory complex concentrated alloy (RCCA) possesses broad application prospects in a wide temperature window owing to its remarkable strength-ductility synergy and super rolling formability, whereas poor oxidation resistance will give rise to excessive wear above 800 ℃. In this contribution, we aim to fabricate a protective silicide coating on VNbTa RCCA for the purposes of enhancing high-temperature oxidation as well as wear resistance. Microstructural characterizations revealed that a uniform and intact coating was obtained under the optimized processing condition of pack cementation. The coating is mainly composed of a single-phase (VNbTa)Si2, along with a thin M5Si3-type interdiffusion zone. Compared with bare alloy, the coated specimens exhibit remarkable oxidation resistance at 1000 ℃ due to the preferential formation of protective SiO2 scale. Meanwhile, the coated specimens maintain desirable wear resistance from 800 ℃ to 1000 ℃. The results of this study would offer a valuable reference to expedite the future engineering applications of RCCAs.

Development of high hard TiB–TiB2 coatings on Ti implants for bio-tribological applications; applying Box-Behnken design and actual wear analyzing in simulated body fluids

This research focuses on the enhancement of the tribological behavior of commercial pure titanium (Cp-Ti) in a simulated body fluid (SBF) via surface reinforcement by titanium borides. The protective boride coatings were formed through the pack cementation process at two different temperatures (950 and 1050 °C) to create diverse hard compositions of TiB and TiB–TiB2 in the coating, with hardness of 1032 and 1291 Hv, respectively. The roughness and wettability of top surfaces were evaluated before the tribology studies due to their impacts on the tribology behavior. Box-Behnken design (BBD) was applied to investigate the effects of the wear parameters in dry and SBF environments. The duration of the wear tests was 53, 35, and 26 min for low, medium, and high speed. It was found that the abrasion wear type was dominant at the beginning of sliding, and then changed to the plastic deformation in the dry conditions, while there is no sign of the abrasion and adhesion wear features at the surface of the reinforced samples abraded in the SBF solution. Results confirmed a high coincidence between BBD predictions and actual findings. The released debris from the coatings was also investigated to find origins. Results showed that majority of the debris in the SBF solution belonged to the calcium phosphate particles demonstrating the bioactivity of the TiB–TiB2 coatings. According to findings, hard TiB–TiB2 coatings could significantly improve the tribological behavior of the Ti implants, which could in turn result in expanding the biological application of Cp-Ti.

Novel Chromium–Silicon Slurry Coatings for Hot Corrosion Environments

Ni-based superalloys are commonly used in gas turbines because of their exceptional high-temperature mechanical properties. To secure a long service life, the materials must also have sufficient corrosion resistance. Therefore, diffusion coatings are widely used to enrich the surface in protective oxide scale-forming elements. For temperatures between 650 and 950 °C, where hot corrosion occurs, Cr-based coatings are advantageous. These are commonly applied via the laborious pack cementation process. Recently, a novel cost-effective Cr/Si slurry coating process has been developed which demonstrated resistance to oxidative high-temperature environments. Here, the protection of the slurry coatings against hot corrosion type I at 900 °C on the Ni-based superalloy Rene 80 is investigated and compared to coatings produced by pack cementation. Prior to the 300-h exposures in air containing 0.1% SO2 at 900 °C, 4 mg/cm2 of Na2SO4 was deposited on the material surfaces. The uncoated Rene 80 exhibited rapid dissolution of the initial oxide scale followed by catastrophic break away oxidation. In comparison, the slurry coatings showed significantly improved hot corrosion resistance compared to the uncoated alloy and a better protection than a Cr pack cementation coating. The Cr pack cemented Rene 80 showed improved hot corrosion resistance, but Cr depletion in the subsurface zone occurred with increasing exposure time, associated with the propagation of Al internal oxidation and increasing sulfidation. In contrast, the slurry coatings formed an external Cr2O3 scale coupled with an agglomeration of SiO2 underneath and a continuous Al2O3 subscale which offered a better diffusion barrier and leading to superior long-term protection against hot corrosion.

In-situ formation and ablation mechanism of dense and multilayered ZrB2-ZrSi2-MoSi2 ultra-high temperature ceramic composite coating on Ta-W alloy under plasma blame beyond 2000 ℃

A dense and three-layered ZrB2-ZrSi2-MoSi2 composite coating with continuous interlayer interfaces and a thickness of ∼270 μm was in-situ prepared on Ta-10 W alloy by plasma spraying and pack cementation. Subjected to plasma ablation at 9.8 MW/m² for 1000 seconds, the coating with a mass ablation rate of 0.0002 mg/s undergoes oxidation to form a skeleton-structured ZrO2 and liquid self-sealing SiO2. During the ablation, the over-diffused silicon top layer of the coating with high silicon content and uniform phase distribution can produce relatively adequate SiO2 to withstand the consumption for up to 1000 seconds, yielding excellent ablation resistance above 2000 ℃.

Wear-resistant enhanced composite coatings on TC4: Combining halide-activated pack cementation and plasma electrolytic oxidation

Enhancing the wear resistance is crucial to broadening titanium alloy application potential. In this study, we discovered that preparing a boron-infiltrated prefabricated coating using halide-activated pack cementation (HAPC) before plasma electrolytic oxidation (PEO) can significantly improve the wear resistance of TC4. The composite coatings (HAPC/PEO) produced through this method exhibit effective boron (B) penetration, with the main phase being H3BO3. We investigated the properties of HAPC/PEO and compared the wear resistance to a single PEO coating. Our results show that the hardness and modulus of elasticity of HAPC/PEO are enhanced, while its resistance to plastic deformation is increased and the coating is not easily damaged during friction. HAPC/PEO shows a 55 % decrease in the coefficient of friction and a 47 % reduction in wear rate compared to PEO coating, indicating superior wear resistance. We analyzed the stress distribution and displacement changes during friction between the PEO coating and HAPC/PEO using ABAQUS. The influence of the main phases' structure on the two coatings' wear resistance is also discussed, and the significant role of the laminated structure of H3BO3 in enhancing the wear resistance of HAPC/PEO was discovered.

Comparison of diffusion coatings resistance on ferritic and austenitic-stainless steels at 650 °C in steam oxidation

Iron-aluminide coatings were deposited on T24, P92 and Incoloy 800HT (IN-800HT) by pack-cementation and by a water-based slurry. According to the differences in heat treatments between the two processes, Al-rich phases, i.e. Fe4Al13 and Fe2Al5 were obtained by pack cementation and only B2-(Fe, Ni)Al by slurry. Vertical or horizontal cracks were present on certain coatings, the former being due to the coefficient of thermal expansion (CTE) mismatch with the substrate while the latter were related to the brittleness of the Al-rich phases. The oxidation behaviour of uncoated and coated substrates was studied at 650 °C up to 1000 h under a low steam flow rate (1.07 × 10−5 m/s). The aluminide coatings were very protective on the low Cr substrates (i.e. T24 and P92) because of the formation of a very thin oxide scale compared to the non-protective thick layers of Fe3O4 and Fe2O3 that grew on both uncoated substrates. In contrast, none of the coatings significantly improved the oxidation resistance of the IN-800HT steel despite the growth of a thin oxide layer made of (Fe, Cr)2O3 and (Fe, Cr)3O4 at 650 °C.

Effect of B on hot corrosion behavior of silicide coatings under different corrosive environments at 900 °C

Silicide coatings and B-modified silicide coatings were prepared on Nb-Si based alloys using halide activated pack cementation. The hot corrosion behavior of the coatings exposed to Na2SO4 and Na2SO4 + NaCl mixture salts at 900 °C was investigated. The results show that B-modified silicide coatings possess better hot corrosion resistance than the silicide coatings. The hot corrosion products of silicide coatings show a regional microstructure, including uplift regions of AlNaSiO4 imbedded with Al2O3 phases, and depressed silica regions with NaNbO3 and TiO2 particles. The addition of B to the silicide coatings promotes the formation of the uniform and dense boroaluminosilicate scale with better fluidity during hot corrosion, which impedes the self-sustaining cyclic chlorination/sulfidation reactions, and thus, improves the hot corrosion resistance of the silicide coatings.

Enhancing high-temperature oxidation resistance of tantalum-tungsten alloy with composite coating

The poor oxidation resistance of tantalum-tungsten alloy at high temperatures limits its development as an ideal candidate for thermal components in aerospace. In this study, a composite coating including an internal TaSi2-WSi2 layer and an external ZrB2-MoSi2-ZrSi2 layer was prepared on the surface of Ta10W alloy by combining slurry sintering and halide activated pack cementation (HAPC). The composite coating has a dense structure, and the coating samples remains intact after oxidation at 1500°C for 6 hours, while the unprotected substrate oxidizes to powder within 3 minutes. The coated samples also withstood more than 300 shock heating cycles from room temperature to 1500°C. Its notable resistance to high-temperature oxidation was ascribed to the development of a compact and uniform SiO2 oxide layer containing oxide crystals comprising ZrSiO4, ZrO2, and Ta2O5 during the oxidation process, which effectively mitigated the diffusion of oxygen into the interior of the coating.

High heat flux ablation behaviors of Ir-Hf coatings in wind tunnel tests up to 6.6 MW/m²

The Ir-Hf coatings have good resistance to oxidative ablation, in order to investigate the ablation resistance limit and ablation failure mechanism of the Ir-Hf coatings, graphite/Re/Ir/Ir-Hf samples were prepared by an optimized pack cementation method. The Ir-Hf coating samples were tested for anti-ablation properties under stepwise and constant working conditions in a high-frequency plasma wind tunnel, respectively. The results showed that the Ir-Hf coating passed the high-frequency plasma wind tunnel test at a heat flux of 5.7 MW/m2 for 650 s without defects. And the heat flux limit of the Ir-Hf coating was about 6.6 MW/m2 in the plasma wind tunnel with a lifetime no less than 400 s and a surface temperature of about 2200℃, exceeding most of the reported heat flux limits for non-ablative materials, such as SiC, ZrB2 and HfC. The failure mechanism of the Ir-Hf coating is as follows: when the surface temperature reaches a certain level, the interface temperature between Ir and the oxide layer will exceed its low eutectic point and a liquid phase will appear, triggering failure. The Ir-Hf coatings have the advantages of high heat flux resistance, zero ablation, low thermal response and good protection of the graphite substrate, which makes them promising as the thermal protection coatings in ultra-high temperature (over 2000 °C) aerospace applications.

Recent Advances in the Deposition of Aluminide Coatings on Nickel-Based Superalloys: A Synthetic Review (2019–2023)

Thermal barrier coatings (TBCs) are widely used to improve the oxidation resistance and high-temperature performance of nickel-based superalloys operating in aggressive environments. Among the TBCs, aluminide coatings (ACs) are commonly utilized to protect the structural parts of jet engines against high-temperature oxidation and corrosion. They can be deposited by different techniques, including pack cementation (PC), slurry aluminizing or chemical vapor deposition (CVD). Although the mentioned deposition techniques have been known for years, the constant developments in materials sciences and processing stimulates progress in terms of ACs. Therefore, this review paper aims to summarize recent advances in the AC field that have been reported between 2019 and 2023. The review focuses on recent advances involving improved corrosion resistance in salty environments as well as against high temperatures ranging between 1000 °C and 1200 °C under both continuous isothermal high-temperature exposure for up to 1000 h and cyclic oxidation resulting from AC application. Additionally, the beneficial effects of enhanced mechanical properties, including hardness, fatigue performance and wear, are discussed.

Corrosion tests on austenitic samples with alumina and alumina-forming coatings in oxygen-containing stagnant Pb and turbulently flowing PbBi

Two differently produced alumina coatings (by pulsed laser deposition (PLD) and detonation gun (DG)) and one alumina-forming coating realised by pack cementation are proposed as a protection barrier against corrosion of austenitic steels in Pb and PbBi. Samples were tested in oxygen-controlled stagnant Pb (10−7 wt.%) at 480 °C and 550 °C for up to 10,000 h and in turbulently flowing (up to 1.6 m/s) PbBi eutectic at 490 °C with 10−9–10−8 wt.% oxygen for about 500 h. All exposed coatings showed a good behaviour in flowing PbBi and in stagnant Pb at around 480 °C independent of the oxygen content. At 550 °C, the PLD coating failed most probably due to incomplete coating of the sample, while the DG sample protected the base material.

Formation Mechanism of Ti–Si Multi-Layer Coatings on the Surface of Ti–6Al–4V Alloy

Titanium alloys are widely used in aerospace applications due to their high specific strength and exceptional corrosion resistance. In this study, a silicide coating with a multi-layer structure was designed and prepared via a pack cementation process to improve the high-temperature oxidation resistance of titanium alloy. A new theory based on the Le Chatelier’s principle is proposed to explain the generation mechanism of active Si atoms. Taking the chemical potential as a bridge, a functional model of the relationship between the diffusion driving force and the change in the Gibbs free energy of reaction diffusion is established. Experimental results indicate that the depth of the silicide coating increases with the siliconization temperature (1000–1100 °C) and time (0–5 h). The multi-layer coating prepared at 1075 °C for 3 h exhibits a thick and dense structure with a thickness of 23.52 μm. This coating consists of an outer layer of TiSi2 (9.40 μm), a middle layer of TiSi (3.36 μm), and an inner layer of Ti5Si3 (10.76 μm). Under this preparation parameter, increasing the temperature or prolonging the holding time will cause the outward diffusion flux of atoms in the substrate to be much larger than the diffusion flux of silicon atoms to the substrate, thus forming pores in the coating. The calculated value of the diffusion driving force FTiSi = 2.012S is significantly smaller than that of FTiSi2 = 13.120S and FTi5Si3 = 14.552S, which perfectly reveals the relationship between the thickness of each layer in the Ti–Si multi-layer coating.

Effects of aluminizing on the microstructure and wear resistance of AISI 321 steel

To enhance the surface properties, particularly hardness and wear resistance, of AISI 321 steel, an aluminizing coating was introduced through pack cementation treatment. Microstructure characterization and wear resistance tests were conducted to elucidate the intricate relationship between the microstructure and wear behaviors. A thorough analysis on the structure and compositions revealed the incorporation of FeAl(Cr) phases, aluminum oxides, titanium compounds and B2-NiAl phases in the coating, which resulted in a remarkable 192 % increase on the surface hardness, elevating it from 226 HV to an impressive 662 HV. In comparison to the uncoated 321 steel, the aluminized counterpart exhibited a pronounced reduction in coefficients of friction (COFs), wear volume losses, and wear rates. The wear mechanisms of both the uncoated 321 steel and aluminized steel were intricately analyzed. The uncoated 321 steel endured severe adhesive and oxidation wear, characterized by pronounced plastic deformation, the presence of wear debris and flaking oxides. In contrast, the aluminized steel predominantly underwent abrasive wear, showcasing a relatively smooth worn surface and dense continuous oxides. These findings not only reveal the significant enhancement of tribological properties by aluminized coatings, but also provide valuable insights into their fundamental behavior during wear.