# State of the Art in Metal Matrix Composites Research: A Bibliometric Analysis
## 1 Introduction
### Bibliometric Studies  
Bibliometric analysis complements the traditional literature reviews by providing an objective evaluation of the trends and future potential of a research area [59; 60; 61; 62]. This analysis is useful to highlight the evolution of a particular research area over a period of years or decades [61]. There are many methods to review the scientific research developments in a particular field [63]. These methods can be categorised under two broad classifications--evaluative and relational reviews [64; 65]. Evaluative reviews generally deal with the quantitative analysis of the absolute research impacts of research articles, authors, organizations and nations. This analysis includes various productivity metrics such as the number of publications, publications per year, number of citations and more [66; 67; 68; 69]. Evaluative reviews can also be conducted qualitatively based on expert opinions of research impact metrics in a particular area [70]. On the other hand, the relational review methodology focuses on inter-relationships among the above-mentioned evaluative impact metrics. Theseinter-relationships are measured and expressed in terms of the number of collaborative outputs, collaborative linkages and respective link strengths. Other relational metrics include citations based linkages, co-occurrences and co-citations based analyses [71]. Co-authorship linkages are important indicators of knowledge sharing among different research groups [72]. Citation linkages are vital for understanding research relationships among productive authors and organizations [73]. Determination of linkage strengths reveals the extent of collaborations/co-citations across geographies [74]. The current study considers both evaluative as well as relational tools for a balanced bibliometric analysis of metal matrix composites research. Bibliometric surveys provide insights into the development of a research area over a time period of one or more decades [59; 75]. These analyses typically yield vital information about research outputs of individual authors and respective affiliating organizations [68; 69; 76]; collaboration and citation networks among documents, authors, organizations and places [76; 77; 78]; trending topics and keywords [70; 79]; top publishing journals, inter-journal linkages [66; 80; 81] and more. The above mentioned bibliometric results can be effectively analysed and displayed by employing visual data representation tools such as VoS viewer [82], CiteSpace [83; 84], ATLAS.ti [85], BiPlot [86], BibExcel [87], Network Workbench [88; 89], VantagePoint [90] and many more. Data visualization tools are vital for effectively communicating the analysis results as well as for quickly arriving at useful conclusions [91; 92]. In the present work, VoSviewer and CiteSpace were utilized for the quantitative data analysis and visualization of MMC research published in WoS during 2001-2020. VoSviewer generates author and journal network maps based on bibliometric analysis of the collaboration/citation data. It can also create keyword maps showing inter-linkages among various co-occurring terms related to a research area [82; 93]. CiteSpace is a Java application designed to analyse and visualize citation based networks [83], co-authorship links, time series analysis of important topics and citation/reference bursts [94]. Recently, few researchers have conducted bibliometric analysis on some aspects of metal matrix composites. [PERSON] and [PERSON] [95] analysed MMCs reinforced with environment friendly silica. [PERSON] et al. [96] studied the tribological properties and applications of composite materials with special emphasis on eastern European nations. [PERSON] et al. [97] used bibliometric analysis to explore the impact of friction stir processing to produce MMCs for industry 4.0 applications. [PERSON] and [PERSON] [98] conducted another bibliometric survey on energy storage applications of polymer matrix composites.