Direct Visualization of Chemical Transport in Solid-State Chemical Reactions by Time-of-Flight Secondary Ion Mass Spectrometry.

Systematic control and design of solid-state chemical reactions are required for modifying materials properties and in novel synthesis. Understanding chemical dynamics at the nanoscale is therefore essential to revealing the key reactive pathways. Herein, we combine focused ion beam-scanning electron microscopy (FIB-SEM) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) to track the migration of sodium from a borate coating to the oxide scale during in situ hot corrosion testing. We map the changing distribution of chemical elements and compounds from 50 to 850 °C to reveal how sodium diffusion induces corrosion. The results are validated by in situ X-ray diffraction and post-mortem TOF-SIMS. We additionally retrieve the through-solid sodium diffusion rate by fitting measurements to a Fickian diffusion model. This study presents a step change in analyzing microscopic diffusion mechanics with high chemical sensitivity and selectivity, a widespread analytical challenge that underpins the defining rates and mechanisms of solid-state reactions.

Influence of Structural Disorders on the Tribological Behavior of Phosphate-Intercalated Layered Double Hydroxide Additives in Polyalphaolefin.

In this work, several phosphate-intercalated Mg-Al layered double hydroxides (LDHs) were synthesized and evaluated as solid lubricant additives in polyalphaolefin (PAO-4) by means of tribotesting on coupled GCr15/cast iron contacts. The effects of test parameters such as normal loads, additive concentrations, and substrate surface roughness were investigated, while the LDH crystal structure received considerable attention. Several types of structural disorder after anion exchange were identified based on X-ray diffraction (XRD) analysis. The unstable structures promote feasible shearing during sliding to improve friction and wear. In addition, antiwear properties correlate well with the anion charge number or the quantity of anion in the interlayer region. Overall, the tribological performance increased in the order HPO42--LDH < PO43--LDH < P2O74--LDH < (PO3)66--LDH. (PO3)66--LDH demonstrated the best antiwear performance with a reduction of 69% of the ball volume loss compared to PAO-4 oil due to the synergy of the disordered stacking LDH sheets and flexible ring structure of the (PO3)66- anion. Furthermore, on polished surfaces, the coefficient of friction (COF) of the (PO3)66--LDH sample dropped significantly by 26%, while the wear loss reduction of more than 80% was also substantial compared to the base oil sample. A performance comparison between the best-performing LDH additive was also conducted against popular nanomaterials, such as hexagonal boron nitride (BN), graphene nanoplatelets (GNPs), and titanium oxide (TiO2). The performance of (PO3)66--LDH was close to that of GNPs.

Intrinsic Effect of Alkali Concentration on Oxidation Reactivity and High-Temperature Lubricity of Silicate Melts between Rubbed Steel/Steel Contacts.

The present study investigated oxidation reactivity and hot lubricity of a sodium silicate melt at different Na2O/SiO2 ratios under elevated temperature stimulation. Static oxidation prevention was achieved at 920 °C when the Na2O/SiO2 ratio reached 1:3 (trisilicate) and 1:2 (disilicate), but it started to deteriorate in the case of 1:1 (metasilicate). At a high concentration of sodium (metasilicate), a severe corrosion reaction between the melt and oxide took place that resulted in a composite coating on the steel substrate. This high-temperature reaction accelerated the formation of ionic charges from the steel base and promoted oxidation. However, friction and wear reduction is proportional to an increase in the sodium oxide fraction. Metasilicate (1:1) exhibited excellent lubricity under the hot frictional test at 920 °C compared to other lubricants. It was due to the formation of the sodium-saturated surfaces and an amorphous silica layer, which was associated with the high-temperature reactivity of sodium toward the oxide surface. In addition, the NaFeO2-Fe2O3 composite film, as the reaction product of individual sodium charge and oxide, plays a significant role in maintaining the tribofilm stability for metasilicate, which was not present for disilicate. This study advances the understanding of how sodium-containing compounds perform oxidation prevention and generate lubricity at hot rubbed surfaces.

Tribochemistry of adaptive integrated interfaces at boundary lubricated contacts.

Understanding how an adaptive integrated interface between lubricant additives and solid contacts works will enable improving the wear and friction of moving engine components. This work represents the comprehensive characterization of compositional and structural orientation at the sliding interface from the perspective of surface/interface tribochemistry. The integrated interface of a lubricant additive-solid resulting from the friction testing of Graphite-like carbon (GLC) and PVD-CrN coated rings sliding against cast iron under boundary lubrication was studied. The results indicate that in the case of the CrN/cast iron pair the antiwear and friction behavior were very strongly dependent upon lubricant. In contrast, the tribology of the GLC surface showed a much lower dependence on lubrication. In order to identify the compounds and their distribution across the interface, x-ray microanalysis phase mapping was innovatively applied and the principle of hard and soft acids and bases (HSAB) to understand the behaviour. Phase mapping clearly showed the hierarchical interface of the zinc-iron polyphosphate tribofilm for various sliding pairs and different sliding durations. This interface structure formed between lubricant additives and the sliding surfaces adapts to the sliding conditions - the term adaptive interface. The current results help explain the tribology of these sliding components in engine.

Superstrength of nanograined steel with nanoscale intermetallic precipitates transformed from shock-compressed martensitic steel.

An increasing number of industrial applications need superstrength steels. It is known that refined grains and nanoscale precipitates can increase strength. The hardest martensitic steel reported to date is C0.8 steel, whose nanohardness can reach 11.9 GPa through incremental interstitial solid solution strengthening. Here we report a nanograined (NG) steel dispersed with nanoscale precipitates which has an extraordinarily high hardness of 19.1 GPa. The NG steel (shock-compressed Armox 500T steel) was obtained under these conditions: high strain rate of 1.2 µs-1, high temperature rise rate of 600 Kµs-1 and high pressure of 17 GPa. The mean grain size achieved was 39 nm and reinforcing precipitates were indexed in the NG steel. The strength of the NG steel is expected to be ~3950 MPa. The discovery of the NG steel offers a general pathway for designing new advanced steel materials with exceptional hardness and excellent strength.

Experiment and molecular dynamics simulation of nanoindentation of body centered cubic iron.

Experiments and molecular dynamics (MD) simulations have been conducted to investigate the nanoindentation behaviours of iron with body centered cubic (BCC) structure. The experiments show that the indentation hardness decreases with the indentation depth and it changes sharply for a small depth. Two cases with different crystallographic orientations have been simulated. The indentation plane is (010) for Case I and (111) for Case II, respectively. The calculated harness (17.4 GPa for Case I and 22.6 GPa for Case II) are in reasonable agreement with the experimental value (24.2 GPa). The simulation results show that the crystallographic orientation significantly influences the indentation deformation. Case I and Case II exhibit different deformation patterns. The indentation force and the hardness in Case I are smaller than Case II. It is also found that the pileup around the indenter is mainly formed along [110] direction for both cases.

Influence of displacement and its first- and second-order derivative components on curvature fringe formations in speckle shearography.

The influence of displacement and its first- and second-order derivative components on curvature fringe formations in speckle shearography is discussed. The results show that (a) all the displacement components have no direct influence on curvature fringe formations; (b) only the first-order derivative component along the centerline of three apertures has an influence on curvature fringe formations, whereas all the other first-order components have no influence; and (c) all the second-order derivative components have no influence on curvature fringe formations. Results from theory and experiments are in good agreement.